Fix warnings/errors for strict aliasing & function prototypes

This fixes various issues related to gcc's strict-aliasing warning
by using unions. It also handles various cases hit with
gcc's missing-declarations warning.

glslang/OSDependent/osinclude.h

@@ -56,6 +56,7 @@ typedef unsigned int (*TThreadEntrypoint)(void*);
 void* OS_CreateThread(TThreadEntrypoint);
 void OS_WaitForAllThreads(void* threads, int numThreads);
 
+void OS_CleanupThreadData(void);
 void OS_Sleep(int milliseconds);
 
 void OS_DumpMemoryCounters();