docs: mention cost of globbing during variable expansion

docs: mention cost of globbing during variable expansion

* doc/autoconf.texi (Shell Substitutions) <${var=literal}>:
Recommend quoting substitutions that might trigger globbing.
(Limitations of Builtins) <:>: Likewise.
* bin/autoconf.as: Follow our own advice.
* lib/autoconf/functions.m4 (AC_FUNC_SELECT_ARGTYPES): Likewise.
* lib/autoconf/general.m4 (_AC_INIT_PARSE_ARGS): Likewise.
* lib/autoconf/status.m4 (AC_OUTPUT): Likewise.
* lib/autotest/general.m4 (_AT_FINISH): Likewise.
* lib/m4sugar/m4sh.m4 (AS_TMPDIR): Likewise.
* tests/autotest.at (parallel autotest and signal handling):
Likewise.
* tests/c.at (AC_OPENMP and C, AC_OPENMP and C++): Likewise.
* tests/foreign.at (shtool): Likewise.
* tests/fortran.at: Likewise.
* tests/tools.at (autom4te preselections): Likewise.
* tests/torture.at (VPATH): Likewise.

Signed-off-by: Eric Blake <eblake@redhat.com>