This conflict with the DJGPP libc math.h which includes a PI macro (to
M_PI).

We cannot make DJGPP POSIX only (using -D_POSIX_SOURCE) to avoid this
kind of symbols conflicts due to the lack of both posix_memalign and
memalign (DJGPP non standard function) in that POSIX mode. We currently
rely on memalign for aligned heap allocation.