BUILD: makefile: add an implicit USE_MATH variable for -lm

BUILD: makefile: add an implicit USE_MATH variable for -lm

Lua and 51d make use of -lm, which would be better served by having its
own option than being passed in the LDFLAGS. It also simplifies linking
against a static version of libm. The option uses its own LDFLAGS which
are automatically collected into OPTIONS_LDFLAGS.