gnulib-tool: In tests directories, augment AM_CFLAGS and AM_CXXFLAGS. Reported by Pádraig Brady in <https://lists.gnu.org/archive/html/bug-gnulib/2025-09/>. * gnulib-tool.sh (func_emit_tests_Makefile_am): Append the warning-protection options to AM_CFLAGS and AM_CXXFLAGS, in addition to initializing CFLAGS and CXXFLAGS. * pygnulib/GLEmiter.py (GLEmiter.tests_Makefile_am): Likewise. * doc/gnulib-tool.texi (Modified build rules): Document the need to initialize AM_CFLAGS and AM_CXXFLAGS in tests directories. * NEWS: Mention the new requirement.