build: do not override gnulib-provided AM_CFLAGS options

build: do not override gnulib-provided AM_CFLAGS options

Avoid a warning from automake:
lib/Makefile.am:24: AM_CFLAGS multiply defined in condition TRUE ...
lib/gnulib.mk:30: ... `AM_CFLAGS' previously defined here
lib/Makefile.am:20:   `lib/gnulib.mk' included from here

* lib/Makefile.am (AM_CFLAGS): Append $(WARN_CFLAGS) and
$(WERROR_CFLAGS), i.e., use "+=", not "=".
This was introduced via 2009-12-17 commit 0341fc22,
"build: with --enable-gcc-warnings, use -Werror",
but fortunately is not a bug, because the definition
it would have overridden was always empty.