libtool: fix Solaris 11 builds

Trying to build clamav on Solaris 11.3 with the Oracle C compiler,
I got the following error:

libtool: error: not configured to extract global symbols from dlpreopened files

I would have expected a build to use dlopen rather than the preopen
fallback so looked for related configure tests that were perhaps
returning the wrong answer.

The global_symbol_pipe being empty seemed a likely culprit.
the last three lines of nm -p on the conftest.o in this test are:

0000000032 T main
0000000016 T nm_test_func
0000000001 C nm_test_var

On Solaris 10, I'd get a D instead of a C. Adding C to the list of
characters in the symcode variable and building again resulted in a
successful build. I've attached a patch to add this C.

Url: https://savannah.gnu.org/patch/?9086

* m4/libtool.m4 (symcode): Add C for solaris.

m4/libtool.m4

@@ -3944,7 +3944,7 @@ osf*)
   symcode='[[BCDEGQRST]]'
   ;;
 solaris*)
-  symcode='[[BDRT]]'
+  symcode='[[BCDRT]]'
   ;;
 sco3.2v5*)
   symcode='[[DT]]'