there is an issue in the OSX SDK stdarg.h header file which causes va_copy not to be defined with -std=c++11, so use clang's fixed intrinsic header instead

diff:

-#if __STDC_VERSION__ >= 199900L || !defined(__STRICT_ANSI__)
+#if __STDC_VERSION__ >= 199900L || __cplusplus >= 201103L || !defined(__STRICT_ANSI__)
 #define va_copy(dest, src)  __builtin_va_copy(dest, src)
 #endif
oclang/ogcc wrapper: check if '-mmacosx-version-min=' is already given through the invocation command