Log message:
Update slibtool to 0.5.13

0.5.13:

This patch release ensures successful installation of a shared library with
which no accompanying static archive was created. The release also includes
two improvements in slibtool's own build system.

- link mode: create an archive /dev/null symlink placeholder as needed.
- PE soname support: added the designated LDFLAGS_SONAME variable.
- build system: configure: pass --sysroot to the linker as a compiler argument.

Log message:
Import slibtool-0.5.12 as devel/slibtool

slibtool is an independent reimplementation of the widely used libtool,
written in C. slibtool is designed to be a clean, fast, easy-to-use
libtool drop-in replacement, and is accordingly aimed at package authors,
distro developers, and system integrators. slibtool maintains compatibility
with libtool in nearly every aspect of the tool's functionality as well as
semantics, leaving out (or turning into a no-op) only a small number of
features that are no longer needed on modern systems.