M_PI and M_PI_2 are not C++ standard and are therefore not present, in
MinGW-w64 gcc 5.1.0 for example. We therefore replace it with an own
definition.
Since math.h was only included for M_PI and M_PI_2 makros, we can now
remove it.
Signed-off-by: Squareys <Squareys@googlemail.com>