Don’t default to Valgrind unless ENABLE_CHECKING

Don’t default to Valgrind unless ENABLE_CHECKING

* src/alloc.c (USE_VALGRIND): If not defined, don’t default it to
1 unless ENABLE_CHECKING.  The Valgrind hooks bloat the garbage
collector a bit in production, and there’s no need for them these
days if one has a Valgrind suppressions file (which one needs anyway).
(mark_maybe_pointer): Use ‘#if USE_VALGRIND’ instead of ‘#ifdef
USE_VALGRIND’ for consistency with other uses of USE_VALGRIND.
This is in case someone builds with ‘-DENABLE_CHECKING
-DUSE_VALGRIND=0’ in CFLAGS.