Don't expose all BSD extensions.

Just expose the ones that bionic historically leaked.

Also, many of the M_* constants in <math.h> are actually POSIX.

Change-Id: I6275df84c5866b872b71f1c8ed14e2aada12b793
diff --git a/libm/Android.bp b/libm/Android.bp
index 25c3e8f..23b9d5e 100644
--- a/libm/Android.bp
+++ b/libm/Android.bp
@@ -510,6 +510,7 @@
 
     cflags: [
         "-D__BIONIC_NO_MATH_INLINES",
+        "-D_BSD_SOURCE",
         "-DFLT_EVAL_METHOD=0",
         "-include freebsd-compat.h",
         "-Werror",