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
diff --git a/tests/math_sincos_test.cpp b/tests/math_sincos_test.cpp
index 0fab2c2..f2e30de 100644
--- a/tests/math_sincos_test.cpp
+++ b/tests/math_sincos_test.cpp
@@ -14,6 +14,8 @@
  * limitations under the License.
  */
 
+#define _GNU_SOURCE 1
+
 #include <math.h>
 
 #include <gtest/gtest.h>