Define the nedmalloc feature configuration macros for nedmalloc.o, only.
This keeps assert(3) working for the rest of the git source; it was
turned off for nedmalloc users before by defining NDEBUG globally.
Also remove -DUSE_NED_ALLOCATOR as this macro isn't used anywhere.
Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx> Signed-off-by: Junio C Hamano <gitster@pobox.com>