Since we take a float parameter and return a float value we will also just
work on float values all the way. This silences some float conversion
warnings.
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.
