* automake.in (check_user_variables): New function, extracted
from ...
(handle_languages): ... here.
(handle_languages, define_compiler_variable, define_link_variable):
Honore LIBTOOLFLAGS.
(handle_single_transform): Check _LIBTOOLFLAGS in
addition to other per-target flags for Libtool objects.
(handle_libtool): Warn if LIBTOOLFLAGS is defined.
* doc/automake.texi (Libtool Flags, Program and Library Variables,
Flag Variables Ordering): Document LIBTOOLFLAGS.
* tests/libtool7.test: Check basic support for LIBTOOLFLAGS.
* tests/libtool8.test: Make sure Automake warns about LIBTOOLFLAGS
definitions.
* tests/subobj9.test: Adjust.