Fix headers for C builds

Fixes #1691

source/backend/CarlaBackend.h

@@ -340,7 +340,7 @@ static constexpr const uint PARAMETER_IS_ENABLED = 0x010;
  */
 static constexpr const uint PARAMETER_IS_AUTOMATABLE = 0x020;
 
-// for backwards compatibility
+/* for backwards compatibility */
 static constexpr const uint PARAMETER_IS_AUTOMABLE = PARAMETER_IS_AUTOMATABLE;
 
 /*!

source/includes/CarlaDefines.h

@@ -76,8 +76,8 @@
 # if __cplusplus >= 201103L || (defined(__GNUC__) && (__GNUC__ * 100 + __GNUC_MINOR__) >= 405 && defined(__GXX_EXPERIMENTAL_CXX0X__)) || __has_extension(cxx_noexcept)
 #  define CARLA_PROPER_CPP11_SUPPORT
 #  if (defined(__GNUC__) && (__GNUC__ * 100 + __GNUC_MINOR__) < 407 && ! defined(__clang__)) || (defined(__clang__) && ! __has_extension(cxx_override_control))
-#   define override // gcc4.7+ only
-#   define final    // gcc4.7+ only
+#   define override /* gcc4.7+ only */
+#   define final    /* gcc4.7+ only */
 #  endif
 # endif
 #endif
@@ -96,6 +96,7 @@
 #else
 # include <stdbool.h>
 # include <stddef.h>
+# define constexpr /* note: constexpr is coming to C soon, but no compilers support it yet */
 #endif
 
 /* Define various string format types */