switch c compiler check to different cmake variable (#1169)

cmake/AwsCFlags.cmake

@@ -259,7 +259,7 @@ function(aws_set_common_properties target)
     if (NOT CMAKE_BUILD_TYPE STREQUAL "Debug")
         # And dont hide symbols on anything pre GCC 5.0 (Visibility support was not great on older compilers and some libraries didnt annotate visibility - 
         # looking at you jni, which does not annotate on gcc less than 4.2. Mixing no annotation and hidden symbols leads to unexpected failures.). 
-        if (NOT (CMAKE_CXX_COMPILER_ID STREQUAL "GNU" AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS "5.0"))
+        if (NOT (CMAKE_C_COMPILER_ID STREQUAL "GNU" AND CMAKE_C_COMPILER_VERSION VERSION_LESS "5.0"))
             set_target_properties(${target} PROPERTIES C_VISIBILITY_PRESET hidden CXX_VISIBILITY_PRESET hidden VISIBILITY_INLINES_HIDDEN ON)
         endif ()
     endif ()