Allow to build a static version of libosl.

Allow to build a static version of libosl.

This adds the new target libosl.a which creates an archive that can
be statically linked into applications. Currently this works only on
ELF-based systems including Linux, FreeBSD and NetBSD, but fails on
MacOS. So it is not built automatically.

Unfortunately, creating libosl.a turned out to be trickier than
expected, because -fvisibility=hidden works only for dynamic libraries.
So we need a different method to hide non-static internal symbols
like make_message() which must not be global (external) because they
might clash with symbols used in the application.

objcopy(1) does the trick but we must provide a list of exceptional
symbols that should not be made local. This list is generated from
the osl.h header file.