Clarify that compiler warnings in the Gnulib tests can be ignored.
* gnulib-tool (func_emit_tests_Makefile_am): Emit overrides for CFLAGS
and CXXFLAGS. Emit a dependency of 'all' on 'all-notice' that prints a
notice.
(func_emit_initmacro_start): Add a second argument. If it is true, emit
code to require gl_CC_ALLOW_WARNINGS and gl_CXX_ALLOW_WARNINGS.
(func_import, func_create_testdir): All callers updated.
* m4/gnulib-common.m4 (gl_CC_ALLOW_WARNINGS, gl_CXX_ALLOW_WARNINGS): New
macros.