Revert "tgmath.h: disable builtins"

This reverts commit d1671548b968103f4df1b80659e60ae1fc5a67b3.

You get following errors while compiling freeswitch:
awgn.c: In function 'awgn_init_dbov':                                                                                   awgn.c:110:5: error: void value not ignored as it ought to be                                                                s->rms = pow(10.0, level/20.0)*32768.0;

Reverting this commit allows to build the code.

include/tgmath.h

@@ -47,7 +47,7 @@
 
 /* 1 if 'type' is a floating type, 0 if 'type' is an integer type.
    Allows for _Bool.  Expands to an integer constant expression.  */
-# if 0 /* __GNUC_PREREQ (3, 1) */
+# if __GNUC_PREREQ (3, 1)
 #  define __floating_type(type) \
   (__builtin_classify_type ((type) 0) == 8 \
    || (__builtin_classify_type ((type) 0) == 9 \