[patch] Use --version-script also on non-Linux systems

[Tijl Coosemans]

To implement -export-symbols(-regex) on non-Linux systems with GNU
ld(1) libtool currently uses the linker flag --retain-symbols-file
but this is more like a fine-grained strip(1) (.symtab table) and
does not affect what symbols are exported (.dynsym table).  On Linux
libtool uses --version-script instead.  The patch copies this to
other systems.

Libtool still uses --retain-symbols-file with other languages than
C, but also on Linux so the patch does not change any of that.

This has been tested on the FreeBSD package building cluster.  It
reduced the number of exported symbols in several libraries.  The
-export-symbols(-regex) test cases also work on FreeBSD now.

lt-expsyms.patch
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