Fix <features.h> (_BSD_SOURCE and _GNU_SOURCE).

<features.h> is supposed to take user-settable stuff like _GNU_SOURCE
and _BSD_SOURCE and turn them into __USE_GNU and __USE_BSD for use in
the C library headers. Instead, bionic used to unconditionally define
_BSD_SOURCE and _GNU_SOURCE, and then test _GNU_SOURCE in the header
files (which makes no sense whatsoever).

Bug: 14659579
Change-Id: Ice4cf21a364ea2e559071dc8329e995277d5b987

libm/include/math.h

@@ -17,6 +17,7 @@
 #ifndef _MATH_H_
 #define	_MATH_H_
 
+#include <features.h>
 #include <sys/cdefs.h>
 #include <limits.h>
 
@@ -462,11 +463,11 @@ long double	truncl(long double);
 
 #endif /* __ISO_C_VISIBLE >= 1999 */
 
-#if defined(_GNU_SOURCE)
+#if defined(__USE_GNU)
 void sincos(double, double*, double*);
 void sincosf(float, float*, float*);
 void sincosl(long double, long double*, long double*);
-#endif /* _GNU_SOURCE */
+#endif /* __USE_GNU */
 
 #pragma GCC visibility pop
 __END_DECLS