Enable some FPU optimizations via standard pragma by default

If GCC -ffast-math is not supported, some floating point optimizations
are still possible through more standard means:

"FP_CONTRACT on" is a subset of -funsafe-math-optimizations.

"FENV_ACCESS off" corresponds to -frounding-math.

"CX_LIMITED_RANGE on" corresponds to -fcx-limited-range, but is not
included as VLC does not use complex numbers anywhere yet.

configure.ac

@@ -964,6 +964,12 @@ AS_IF([test "${enable_optimizations}" != "no"], [
     ])
   ])
   AS_IF([test "${ac_cv_c_fast_math}" = "no"], [VLC_RESTORE_FLAGS])
+  AH_BOTTOM([
+#ifndef __FAST_MATH__
+# pragma STDC FENV_ACCESS OFF
+# pragma STDC FP_CONTRACT ON
+#endif
+])
 
   dnl Check for -funroll-loops
   VLC_SAVE_FLAGS