Fix util/mkinstallvars.pl to treat LIBDIR and libdir correctly

Fix util/mkinstallvars.pl to treat LIBDIR and libdir correctly

OpenSSL's build file (Makefile) handles library directories via two
variables, 'LIBDIR' and 'libdir', where the former is empty when the
path given through ./Configure's '--libdir' is absolute.

This was forgotten when treating the resulting values in,
util/mkinstallvars.pl, which got libdir in exporters/libcrypto.pc
to not be quite right if .Configure was called with a '--libdir'
with an absolute path.

The fix turns out to be quite easy.

Resolves: https://github.com/openssl/openssl/issues/28779

Reviewed-by: Viktor Dukhovni <viktor@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/29540)