Fix <math.h> to quieten most of our warnings.

I've reported the wcsftime bug upstream, but we really just want to use -D
to ensure the buggy code isn't built. (I've also brought our strftime a bit
closer to upstream now we have the right define.)

I don't think upstream is likely to fix all their sign-compare and
uninitialized warnings, so let's just silence them.

As for libm, again upstream isn't likely to fix all their warnings, and
silencing those made the ones that were our fault stand out. I've fixed
our <math.h> to fix the warnings caused by our lack of definitions for
the non-imprecise long-double functions. I checked the C99 standard, and
all these functions are there.

Change-Id: Iee8e1182c1db375058fb2c451eceb212bab47a37

tests/math_test.cpp

@@ -14,8 +14,6 @@
  * limitations under the License.
  */
 
-#define _DECLARE_C99_LDBL_MATH 1
-
 // This include (and the associated definition of __test_capture_signbit)
 // must be placed before any files that include <cmath> (gtest.h in this case).
 //