[clang][Headers] Only define FLT_EVAL_METHOD for C99 and later

It was reported by glibc conform test [1].

[1] https://sourceware.org/git/?p=glibc.git;a=blob;f=conform/data/float.h-data;h=7b98fc03447b8918da4a0cf47d41fb3e17f4f791;hb=HEAD

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D137267

Author: zatrazz

clang/lib/Headers/float.h

@@ -86,7 +86,10 @@
 
 /* Characteristics of floating point types, C99 5.2.4.2.2 */
 
+#if (defined(__STDC_VERSION__) && __STDC_VERSION__ >= 199901L) ||              \
+    (defined(__cplusplus) && __cplusplus >= 201103L)
 #define FLT_EVAL_METHOD __FLT_EVAL_METHOD__
+#endif
 #define FLT_ROUNDS (__builtin_flt_rounds())
 #define FLT_RADIX __FLT_RADIX__