build-sys: Determine GNUTLS_LIBS using pkg-config [OS X]

Determine GNUTLS_LIBS using pkg-config rather than hard-coding
it. On OS X it may return a -L<path> to succeed the linking.

Resolves: https://github.com/stefanberger/swtpm/issues/676
Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
1 file changed