Fix -export-symbols and -export-symbols-regex support on Solaris 11.3.
Apply fix from <https://savannah.gnu.org/patch/?9467>.
On Solaris 11.3 (as opposed to Solaris 11.0), the output of /usr/bin/nm -p
on object files contains, for global variables without initializer, the
letter 'C' (it was 'D' in Solaris 11.0).
* m4/libtool.m4 (_LT_CMD_GLOBAL_SYMBOLS): Adjust symcode for Solaris.
