[FEATURE] CMake build system should propagate CMAKE_C_FLAGS to more third-party dependencies

[InspectorConstructor]

The CMAKE_C_FLAGS aren't propagated to all third-party libraries. This makes it impossible to optimize the build for specific architectures without patch-ing files. I would like to use this variable to pass platform-specific flags (specifically -mcpu, -mfpu, -ffunction-sections, -fdata-sections, and -Os .)

All CMake-based dependencies can simply forward the flags with -DCMAKE_C_FLAGS=${CMAKE_C_FLAGS}, and I believe that openssl's configure command can accept C flags as bare arguments.

--- a/CMake/Dependencies/libsrtp-CMakeLists.txt                                                                                                                                          
+++ b/CMake/Dependencies/libsrtp-CMakeLists.txt                                                                                                                                          
@@ -46,13 +48,14 @@ include(ExternalProject)
 ExternalProject_Add(project_libsrtp                                                                                                                                                     
     GIT_REPOSITORY    https://github.com/cisco/libsrtp.git                                                                                                                              
     GIT_TAG           bd0f27ec0e299ad101a396dde3f7c90d48efc8fc                                                                                                                          
     PATCH_COMMAND     ${PATCH_COMMAND}                                                                                                                                                  
     PREFIX            ${CMAKE_CURRENT_BINARY_DIR}/build                                                                                                                                 
     CMAKE_ARGS        -D CMAKE_BUILD_TYPE:STRING=${CMAKE_BUILD_TYPE}                                                                                                                    
                       -D CMAKE_INSTALL_PREFIX:STRING=${OPEN_SRC_INSTALL_PREFIX}                                                                                                         
                       -D ENABLE_OPENSSL=${LIBSRTP_ENABLE_OPENSSL}                                                                                                                       
                       -D BUILD_SHARED_LIBS=${LIBSRTP_SHARED_LIBS}                                                                                                                       
                       -D OPENSSL_ROOT_DIR=${OPEN_SRC_INSTALL_PREFIX}                                                                                                                    
+                      -D CMAKE_C_FLAGS=${CMAKE_C_FLAGS}                                                                                                                                 
     CONFIGURE_COMMAND ${CONFIGURE_COMMAND}                                                                                                                                              
     TEST_COMMAND      ""                                                                                                                                                                
 )                                                                                                                                                                                       
diff --git a/CMake/Dependencies/libusrsctp-CMakeLists.txt b/CMake/Dependencies/libusrsctp-CMakeLists.txt                                                                                 
index bae5c8a..2d66854 100644                                                                                                                                                            
--- a/CMake/Dependencies/libusrsctp-CMakeLists.txt                                                                                                                                       
+++ b/CMake/Dependencies/libusrsctp-CMakeLists.txt                                                                                                                                       
@@ -8,7 +8,7 @@ ExternalProject_Add(project_libusrsctp
     GIT_REPOSITORY    https://github.com/sctplab/usrsctp.git                                                                                                                            
     GIT_TAG           1ade45cbadfd19298d2c47dc538962d4425ad2dd                                                                                                                          
     PREFIX            ${CMAKE_CURRENT_BINARY_DIR}/build                                                                                                                                 
-    CMAKE_ARGS        -DCMAKE_INSTALL_PREFIX=${OPEN_SRC_INSTALL_PREFIX} -DCMAKE_C_FLAGS=-fPIC -Dsctp_werror=0                                                                           
+    CMAKE_ARGS        -DCMAKE_INSTALL_PREFIX=${OPEN_SRC_INSTALL_PREFIX} -DCMAKE_C_FLAGS=${CMAKE_C_FLAGS} -Dsctp_werror=0 -DCMAKE_POSITION_INDEPENDENT_CODE=ON                           
     BUILD_ALWAYS      TRUE                                                                                                                                                              
     TEST_COMMAND      ""                                                                                                                                                                
 )                                                                                                                                                                                       

and for openssl:

-    SET(CONFIGURE_COMMAND ${CMAKE_CURRENT_BINARY_DIR}/build/src/project_libopenssl/config ${OPENSSL_EXTRA} --prefix=${OPEN_SRC_INSTALL_PREFIX} --openssldir=${OPEN_SRC_INSTALL_PREFIX})
+    SET(CONFIGURE_COMMAND ${CMAKE_CURRENT_BINARY_DIR}/build/src/project_libopenssl/config ${OPENSSL_EXTRA} --prefix=${OPEN_SRC_INSTALL_PREFIX} --openssldir=${OPEN_SRC_INSTALL_PREFIX} ${CMAKE_C_FLAGS})