Roll back -ffast-math

Summary:
@public

`-ffast-math` does not have measurable performance benefits.

By using `NaN` for *undefined* values again, we can squeeze `YGFloatOptional` into 32 bits.
This will also enable us to store `YGValue` (or a variant of it) in 32 bits.

Reviewed By: astreet

Differential Revision: D13403925

fbshipit-source-id: b13d026bf556f24ab4699e65fb450af13a70961b

yoga/Yoga.h

@@ -17,14 +17,13 @@
 #include <stdbool.h>
 #endif
 
-/** Large positive number signifies that the property(float) is undefined.
- *Earlier we used to have YGundefined as NAN, but the downside of this is that
- *we can't use -ffast-math compiler flag as it assumes all floating-point
- *calculation involve and result into finite numbers. For more information
- *regarding -ffast-math compiler flag in clang, have a look at
- *https://clang.llvm.org/docs/UsersManual.html#cmdoption-ffast-math
- **/
-#define YGUndefined 10E20F
+// Not defined in MSVC++
+#ifndef NAN
+static const unsigned long __nan[2] = {0xffffffff, 0x7fffffff};
+#define NAN (*(const float*)__nan)
+#endif
+
+#define YGUndefined NAN
 
 #include "YGEnums.h"
 #include "YGMacros.h"