build: fix ADD_CXX_COMPILER_FLAG_IF_AVAILABLE usages

This patch solves two different issues at the same time when building
with gcc/g++ 9.2.0:

 1) The -fvisibility=hidden support check was done using the C++
 compiler only (with check_cxx_compiler_flag), inside the block
 corresponding to the C compiler being GNU/clang, and the result of
 the check was applied to both C++ (CXX_FLAGS) and C (C_FLAGS) flags.
 Instead of this, there should be separate checks for the C and C++
 compilers, each of them modifying a single set of <LANG>_FLAGS.

 2) -Wincompatible-pointer-types support check was done using the C++
 compiler only, and the result of the check was applied to both C++
 (CXX_FLAGS) and C (C_FLAGS) flags. But, this warning is not
 applicable to C++ and actually breaks compilation when using g++ 9.2:
    [  0%] Building CXX object lib/src/asn1/CMakeFiles/rrc_asn1.dir/rrc_asn1.cc.o
    cc1plus: error: ‘-Werror=’ argument ‘-Werror=incompatible-pointer-types’ is not valid for C++ [-Werror]
 Instead of this, there should be a check for this warning only using
 the C compiler, and therefore only modifying the C flags (C_FLAGS).

This patch splits the macro into one specific for C++ (which modifies
CXX_FLAGS) and one specific for C (which modifies C_FLAGS). And so,
the macro call to check for `-Werror=incompatible-pointer-types' is
made C-only, and the one for `-fvisibility=hidden` is done for both C
and C++ targets (each on the correct GNU/clang compiler block).

Due to how the tests are done in cmake, the '-fvisibility=hidden'
check wasn't even succeding before, so the compiler option wasn't
being effectively used. The cmake flags.make file contents throughout
the project are updated as follows now:

Before this change, we had:
   C_FLAGS   = -Werror=incompatible-pointer-types ...
   CXX_FLAGS = -Werror=incompatible-pointer-types ...

And after this change, we have:
   C_FLAGS   = -Werror=incompatible-pointer-types -fvisibility=hidden ...
   CXX_FLAGS = -fvisibility=hidden ...

CMakeLists.txt

@@ -275,10 +275,18 @@ macro(ADD_CXX_COMPILER_FLAG_IF_AVAILABLE flag have)
     include(CheckCXXCompilerFlag)
     check_cxx_compiler_flag(${flag} ${have})
     if(${have})
-        add_definitions(${flag})        
+        set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${flag}")
     endif(${have})
 endmacro(ADD_CXX_COMPILER_FLAG_IF_AVAILABLE)
 
+macro(ADD_C_COMPILER_FLAG_IF_AVAILABLE flag have)
+    include(CheckCCompilerFlag)
+    check_c_compiler_flag(${flag} ${have})
+    if(${have})
+        set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${flag}")
+    endif(${have})
+endmacro(ADD_C_COMPILER_FLAG_IF_AVAILABLE)
+
 if(CMAKE_CXX_COMPILER_ID MATCHES "GNU" OR CMAKE_CXX_COMPILER_ID MATCHES "Clang")
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Wno-comment -Wno-reorder -Wno-unused-but-set-variable -Wno-unused-variable -Wtype-limits -std=c++11")
 
@@ -292,9 +300,13 @@ if(CMAKE_CXX_COMPILER_ID MATCHES "GNU" OR CMAKE_CXX_COMPILER_ID MATCHES "Clang")
       set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -march=${GCC_ARCH} -mfpmath=sse -msse4.1 -DLV_HAVE_SSE")
     endif(HAVE_AVX)
   endif (HAVE_AVX2)
+
+  if(NOT WIN32)
+      ADD_CXX_COMPILER_FLAG_IF_AVAILABLE(-fvisibility=hidden HAVE_VISIBILITY_HIDDEN_CXX)
+  endif(NOT WIN32)
 endif(CMAKE_CXX_COMPILER_ID MATCHES "GNU" OR CMAKE_CXX_COMPILER_ID MATCHES "Clang")
 
-ADD_CXX_COMPILER_FLAG_IF_AVAILABLE("-Werror=incompatible-pointer-types" HAVE_ERROR_INCOMPATIBLE)
+ADD_C_COMPILER_FLAG_IF_AVAILABLE("-Werror=incompatible-pointer-types" HAVE_ERROR_INCOMPATIBLE)
 
 if(CMAKE_C_COMPILER_ID MATCHES "GNU" OR CMAKE_C_COMPILER_ID MATCHES "Clang")
   set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wall -Wno-comment -Wno-write-strings -Winline -Wno-unused-result -Wformat -Wmissing-field-initializers -Wtype-limits -std=c99 -D_GNU_SOURCE")
@@ -363,7 +375,7 @@ if(CMAKE_C_COMPILER_ID MATCHES "GNU" OR CMAKE_C_COMPILER_ID MATCHES "Clang")
   endif(NOT HAVE_SSE AND NOT HAVE_NEON AND NOT DISABLE_SIMD)
 
   if(NOT WIN32)
-      ADD_CXX_COMPILER_FLAG_IF_AVAILABLE(-fvisibility=hidden HAVE_VISIBILITY_HIDDEN)
+      ADD_C_COMPILER_FLAG_IF_AVAILABLE(-fvisibility=hidden HAVE_VISIBILITY_HIDDEN_C)
   endif(NOT WIN32)
 
   if (ENABLE_ASAN AND ENABLE_MSAN)