guile-ncurses-3.0: Undefined references on Gentoo with slibtool

[orbea]

Hi,

Gentoo Bug: https://bugs.gentoo.org/843416
Slibtool: https://dev.midipix.org/cross/slibtool

When building guile-ncurses-3.0 on Gentoo with slibtool it will fail
with many undefined references. This happens for two reasons:

1. On Gentoo both -lncursesw and -ltinfow are needed where the build
   omits -ltinfow/-ltinfo entirely.

2. The build system uses -no-undefined, but with GNU libtool this is
   silently ignored while slibtool will respect the linker flag.

I attached a basic patch against guile-ncurses-3.0 that solves the
issue for me, but feel free to improve upon it if needed.

guile-ncurses-gentoo-slibtool-undefined.patch
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