Remove majority of warnings

There were a *lot* of warnings, managed to remove most of them.

Also, put warning flags before C_FLAGS and CXX_FLAGS, rather than after,
as -Wall -Wextra was overwriting flags that came before it.

libobs/util/c99defs.h

@@ -21,6 +21,8 @@
  * bool, inline, stdint
  */
 
+#define UNUSED_PARAMETER(param) (void)param
+
 #ifdef _MSC_VER
 #define FORCE_INLINE __forceinline
 #else