bug#21076: dynamic-link often fails to load libraries

[Frank Webster <f.webster@yandex.com>]

On my Debian system, `dynamic-link' often fails to load shared
libraries:

Loading using a versioned soname never works:

scheme@(guile-user)> (dynamic-link "libGL.so.1")
ERROR: In procedure dynamic-link:
ERROR: In procedure dynamic-link: file: "libGL.so.1", message: "file not found"

Loading without a library extension doesn't work unless the
corresponding *-dev package is installed:

scheme@(guile-user)> (dynamic-link "libGL")
ERROR: In procedure dynamic-link:
ERROR: In procedure dynamic-link: file: "libGL", message: "file not found"

It is supposed to work with or without the extension, however because
of limitations in the underlying lt_dlopenext function in libtool,
neither works.

The first doesn't work because contrary to its
documentation, lt_dlopenext doesn't always try loading
the bare filename first, without appending an extention.

The second doesn't work because the versioned soname extension (.so.1)
isn't among the ones that lt_dlopenext tries to append. It does try the
unversioned extension (.so), but in Debian the unversioned symlink
is only available when the corresponding *-dev package is installed.

Here is a related libtool bug report: https://debbugs.gnu.org/8976

Note that there are arguably two distinct issues:
(1) lt_dlopenext not trying the bare filename in all cases
(2) lt_dlopenext not trying versioned soname extensions.

Obviously one way to address this would be to fix these two issues in
lt_dlopenext in libtool.

Alternatively, the (system foreign) module in guile could provide a
simple low-level wrapper around lt_dlopen, and possibly implement
the higher level functionality of `dynamic-link' (additional search
paths, guessing extensions etc.) in scheme.