In gdb/acinclude.m4 'libtool' is hard coded within the provided custom
CC variable which results in a failure when using slibtool instead of
GNU libtool where the two different libtool implementations are
incompatible when both are used during the same build process at once.

This can be solved by using the default value for CC instead where
autotools is capable of providing an adequate value by default. I
attached a patch doing this.

A bug report was issued here:
https://sourceware.org/bugzilla/show_bug.cgi?id=30295

Please see comment 3 with the explanation where before I was missing
the real issue.