runtime: allow combined CSMITH_MINIMAL and USE_MATH_MACROS*
runtime: allow combined CSMITH_MINIMAL and USE_MATH_MACROS*

On the one hand, CSMITH_MINIMAL allows using csmith_minimal.h instead
of csmith.h, thus reducing the size of includes in Csmith-generated
files (in particular by avoiding the inclusion of too many standard
header files.)

On the other hand, USE_MATH_MACROS allows replacing functions defined
in safe_math.h with macros (which reduces the size of includes in
Csmith-generated files, in particular if they were generated using
--no-safe-math).

Previously, CSMITH_MINIMAL used to disable USE_MATH_MACROS. Now,
thanks to this commit, both preprocessing/compilation symbols can be
used together and their effects combined.

For more details, see the discussion on the csmith-dev mailing list,
starting from
http://www.flux.utah.edu/listarchives/csmith-dev/msg00498.html

natgla/csmith,Panos90/csmith,natgla/csmith,Panos90/csmith,Panos90/csmith,Panos90/csmith,natgla/csmith,natgla/csmith