Don't use _FPU_GETCW if it won't do what we want

- _FPU_GETCW is defined for more than just x87. Don't use it if the
  control word for the target architecture doesn't support _FPU_EXTENDED
  or _FPU_DOUBLE defined, e.g. pretty much anything but x87. If I had been
  using glibc on PowerPC instead of Apple's libc, I probably would have
  noticed this sooner, since _FPU_GETCW is part of glibc.

src/d_main.cpp

@@ -1973,7 +1973,7 @@ static void D_DoomInit()
 	// Set the FPU precision to 53 significant bits. This is the default
 	// for Visual C++, but not for GCC, so some slight math variances
 	// might crop up if we leave it alone.
-#if defined(_FPU_GETCW)
+#if defined(_FPU_GETCW) && defined(_FPU_EXTENDED) && defined(_FPU_DOUBLE)
 	{
 		int cw;
 		_FPU_GETCW(cw);