COMMON: Move definition of math constants to scummsys header.

This should have no effect on common/math.h as scummsys is included at
the top anyway, but this will fix compilation on some toolchains when
strict ANSI language compliance is enabled i.e. -std=c++11, rather than
-std=gnu11.

This causes an issue as a side effect is to set a guard which disables
various non-standard defines in the system headers i.e. the M_PI and
M_SQRT1_2 constant definitions in the MinGW x86_64 toolchain on
buildbot.

By moving this into scummsys.h after the point of system math.h
inclusion, we can ensure that M_PI etc. are always present.
COMMON: Add comment explaining reason for defining math constants.
Merge pull request #363 from digitall/EnsurePI

Fix compilation with C++11 enabled on Win64 x86_64 toolchain... and probably others.