[Headers][Darwin] Allow #include_next<float.h> to work on Darwin prior to 10.7

This fixes PR31504 and it's a follow up from adding #include_next<float.h>
for Darwin in r289018.

rdar://problem/29856682

llvm-svn: 309752

Author: bcardosolopes

clang/lib/Headers/float.h

@@ -33,6 +33,15 @@
  */
 #if (defined(__APPLE__) || (defined(__MINGW32__) || defined(_MSC_VER))) && \
     __STDC_HOSTED__ && __has_include_next(<float.h>)
+
+/* Prior to Apple's 10.7 SDK, float.h SDK header used to apply an extra level
+ * of #include_next<float.h> to keep Metrowerks compilers happy. Avoid this
+ * extra indirection.
+ */
+#ifdef __APPLE__
+#define _FLOAT_H_
+#endif
+
 #  include_next <float.h>
 
 /* Undefine anything that we'll be redefining below. */