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.