Win64: Only use __builtin_setjmp/__builtin_longjmp when compiling…

… with GCC

Using `__builtin_setjmp` when compiling with Clang on MSYS2 will result in compiler warnings and runtime crashes.  It seems to work on GCC, though.

Compiler warning on Clang:

warning: incompatible pointer types passing 'jmp_buf' (aka 'struct _SETJMP_FLOAT128[16]') to parameter of type 'void **' [-Wincompatible-pointer-types]
                NS_DURING
                ^~~~~~~~~
note: expanded from macro 'NS_DURING'
                    if (!setjmp(NSLocalHandler.jumpState)) {
                                ^~~~~~~~~~~~~~~~~~~~~~~~
note: expanded from macro 'setjmp'
#define setjmp(X)       __builtin_setjmp(X)
                                         ^
1 warning generated.

Headers/Foundation/NSException.h

@@ -54,7 +54,7 @@
 #include <setjmp.h>
 #include <stdarg.h>
 
-#if	defined(__WIN64__)
+#if	defined(__WIN64__) && defined(__GNUC__)
 /* This hack is to deal with the fact that currently (June 2016) the
  * implementation of longjmp in mingw-w64  sometimes crashes in msvcrt.dll
  * but the builtin version provided by gcc seems to work.