configure: test for -Wformat-* w/-Wformat present
configure: test for -Wformat-* w/-Wformat present

TEST_AND_ADD_CFLAGS tests flags independently from AM_CFLAGS. The lack
of -Wformat may cause -Wformat-* checks to fail (gcc 4.9), though in the
end they would have succeeded with -Wall present.

Change-Id: Ic8f0d5b8a3f75a0b105bdc4ddd16690878a30222

webmproject/libwebp,PKRoma/libwebp,PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp,PKRoma/libwebp,PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp