gprim/geom2d.cpp includes mystdlib.h, which already has a fallback define
for M_PI. As geomfuncs.cpp also includes mystdlib.h, use M_PI instead
of a truncated value.
occ/Partition_Loop2d.cxx already gets M_PI from the opencascade headers
(~everything includes Standard_Real.hxx, which includes Standard_math.hxx,
which sets _USE_MATH_DEFINES for Windows and includes math.h).