The intent in bug 1271829 was to make all the XCFLAGS set in
config/external/nss/Makefile.in accumulate instead of replacing each
other. Unfortunately, there is also an AC_SUBST variable named XCFLAGS,
use for other purposes, and we ended up adding on top of that variable.
Using a different local variable allows to still accumulate the values,
while keeping away from the XCFLAGS from autoconf.mk.