libtool: Don't fall back to static libraries if building them was disabled

libtool: Don't fall back to static libraries if building them was disabled

If -no-undefined was not specified but the platform didn't
support shared libraries with undefined symbols (e.g. Cygwin/
MinGW), static libraries were built instead of shared ones,
even if building them was explicitly disabled with --disable-
static configure option.

Fix this by stopping with a fatal error if a shared library
can't be built in this case instead of unexpectedly building a
static library instead.

* libltdl/config/ltmain.m4sh (func_mode_link()): Stop with
fatal error when trying to build a shared library without -no-
undefined on a platform not supporting undefined symbols in
shared libraries.

Copyright-paperwork-exempt: Yes