The _GLIBCXX_USE_C99_MATH macro is defined in glibcxx by c++config.h, which
gets included by every header. Which means a header needs to be present.
(cherry picked from commit 79e9aff338)
This is the case with uClibc-ng currently.
Signed-off-by: Rosen Penev <rosenp@gmail.com>
(cherry picked from commit 769cd0ee9f0cf8ceb026aa751b5d4a390bb5dbdc)
(changed define to match master)
