build: --enable-gcc-warnings: enable -Wstrict-overflow in src/

build: --enable-gcc-warnings: enable -Wstrict-overflow in src/

* configure.ac (WARN_CFLAGS): Don't turn off -Wstrict-overflow.
(GNULIB_WARN_CFLAGS): Remove -Wstrict-overflow from the list of
warning options used in lib/.
Normally I find that -Wstrict-overflow produces too many false
positives, but considering that it warns of the bug reported in
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33498, I now think
it is worthwhile.  The lesser of two evils.
Thanks to Daniel Veillard for showing me the gcc bug report.