Explicitly #define isnan __builtin_isnan for Android clang-5 to mimic gcc's default behavior

Summary:
GCC internally aliases isnan and a dozen other math.h primitives to their `__builtin_` counterparts, which compile straight to assembler. Clang does not do this, so calls to `isnan` compile to a function call. The Android libc does not define `isnan` because it assumes all application code compiled `__builtin_isnan` and has no need for the function `isnan`.

This is a temporary kludge: Starting with NDK r16, the math.h header has appropriate `#defines` to alias primitives to their `__builtin_` counterparts. Upon upgrade to NDK r16, this can be reverted.

Reviewed By: passy

Differential Revision: D6527499

fbshipit-source-id: 65d9e43c7e7d973a6c92e9863ebe469b35a24ac8

yoga/Yoga.cpp

@@ -133,6 +133,13 @@ static int YGDefaultLog(const YGConfigRef config,
 #endif
 
 bool YGFloatIsUndefined(const float value) {
+// TODO(gkm): Ugh! Some Android builds (r13b & clang-3.8) fail
+// with the kludge below, so we must tailor it specifically for
+// NDK r15c which has clang-5.0. NDK r16 will make it all better.
+#if __ANDROID__ && __clang_major__ == 5 // TODO(gkm): remove for NDK >= 16
+#undef isnan
+#define isnan __builtin_isnan
+#endif
   return isnan(value);
 }