Reintroduce M_PI compatibility
Reintroduce M_PI compatibility

M_PI is not part of C99 or C++ and thus aren't provided in all configurations.
This used to be handled in reference_math.c directly but was recently removed.
Just tweak the M_PI handling in the compatibility headers to restore builds.

KhronosGroup/OpenCL-CTS,KhronosGroup/OpenCL-CTS,KhronosGroup/OpenCL-CTS,KhronosGroup/OpenCL-CTS