Fix GnuTLS pkg-config check

Fix GnuTLS pkg-config check

The testlogic for $PKG_CONFIG was inverted, checking for an empty string
before using PKG_CHECK_MODULES. Use -x instead of -z and add an else branch
to the if checking for $GNUTLS_FOUND = no to add -lgcrypt in case the
GnuTLS libraries are detected by pkg-config.