libtool: port to GNU grep 3.8

libtool: port to GNU grep 3.8

GNU grep 3.8 warns about some regular expressions that POSIX says have
undefined effect, e.g., '\-'.  Unfortunately Libtool uses regular
expressions of this form.  Some unittests now fail, e.g. link-order.at:

--- /dev/null
+++ .../libtool/tests/testsuite.dir/at-groups/66/stderr
@@ -0,0 +1,4 @@
+/bin/grep: warning: stray \ before /
+/bin/grep: warning: stray \ before /
+/bin/grep: warning: stray \ before /
+/bin/grep: warning: stray \ before /

Url: https://savannah.gnu.org/patch/index.php?10282

* m4/libtool.m4 (_LT_LANG_CXX_CONFIG): Do not use \- in a BRE or ERE,
as this produces undefined results that GNU grep 3.8 warns about.
Use [-] instead.
* tests/link-order.at (Link order test): Similarly, do not use
\/ in an ERE; use / instead.