diff --git a/libc/bionic/clone.cpp b/libc/bionic/clone.cpp
index 0a0fdd5..9b5c9e7 100644
--- a/libc/bionic/clone.cpp
+++ b/libc/bionic/clone.cpp
@@ -26,7 +26,7 @@
  * SUCH DAMAGE.
  */
 
-#define __GNU_SOURCE 1
+#define _GNU_SOURCE 1
 #include <sched.h>
 #include <stdlib.h>
 #include <stdarg.h>
diff --git a/libc/include/features.h b/libc/include/features.h
index 343c84d..057d1de 100644
--- a/libc/include/features.h
+++ b/libc/include/features.h
@@ -25,18 +25,17 @@
  * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  * SUCH DAMAGE.
  */
+
 #ifndef _FEATURES_H_
 #define _FEATURES_H_
 
-/* certain Linux-specific programs expect a <features.h> header file
- * that defines various features macros
- */
+#if defined(_BSD_SOURCE)
+# define __USE_BSD 1
+#endif
 
-/* we do include a number of BSD extensions */
-#define  _BSD_SOURCE  1
-
-/* we do include a number of GNU extensions */
-#define  _GNU_SOURCE  1
+#if defined(_GNU_SOURCE)
+# define __USE_GNU 1
+#endif
 
 /* C95 support */
 #undef __USE_ISOC95
diff --git a/libc/include/sched.h b/libc/include/sched.h
index e43b6cc..7624950 100644
--- a/libc/include/sched.h
+++ b/libc/include/sched.h
@@ -28,6 +28,7 @@
 #ifndef _SCHED_H_
 #define _SCHED_H_
 
+#include <features.h>
 #include <sys/cdefs.h>
 #include <sys/time.h>
 
@@ -52,7 +53,7 @@
 extern int sched_getparam(pid_t, struct sched_param*);
 extern int sched_rr_get_interval(pid_t, struct timespec*);
 
-#ifdef _GNU_SOURCE
+#if defined(__USE_GNU)
 
 extern int clone(int (*)(void*), void*, int, void*, ...);
 extern int unshare(int);
@@ -146,7 +147,7 @@
 
 extern int __sched_cpucount(size_t setsize, cpu_set_t* set);
 
-#endif /* _GNU_SOURCE */
+#endif /* __USE_GNU */
 
 __END_DECLS
 
diff --git a/libc/include/unistd.h b/libc/include/unistd.h
index 781fc44..c3e655e 100644
--- a/libc/include/unistd.h
+++ b/libc/include/unistd.h
@@ -28,6 +28,7 @@
 #ifndef _UNISTD_H_
 #define _UNISTD_H_
 
+#include <features.h>
 #include <stddef.h>
 #include <sys/cdefs.h>
 #include <sys/types.h>
@@ -112,7 +113,7 @@
 extern int fchdir(int);
 extern int rmdir(const char *);
 extern int pipe(int *);
-#ifdef _GNU_SOURCE
+#if defined(__USE_GNU)
 extern int pipe2(int *, int);
 #endif
 extern int chroot(const char *);
@@ -143,7 +144,7 @@
 
 extern int dup(int);
 extern int dup2(int, int);
-#ifdef _GNU_SOURCE
+#if defined(__USE_GNU)
 extern int dup3(int, int, int);
 #endif
 extern int fcntl(int, int, ...);
diff --git a/libm/include/math.h b/libm/include/math.h
index 4faec33..a808d8d 100644
--- a/libm/include/math.h
+++ b/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 @@
 
 #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
diff --git a/libm/sincos.c b/libm/sincos.c
index ad75549..a5608cf 100644
--- a/libm/sincos.c
+++ b/libm/sincos.c
@@ -22,8 +22,8 @@
  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  * SUCH DAMAGE.
- *
  */
+
 #define _GNU_SOURCE 1
 #include <math.h>
 
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>