Compile problems with >=gnutls-2.7.0

[Bernard Cafarelli]

=gnutls-2.7.0 no longer defines the macro AM_PATH_LIBGNUTLS or distributes gnutls-config that filezilla uses to identify version (via libgnutls.m4).

pkg-config is the replacement that has been distributed by gnutls at least in
version 2.2.5.

I'll attach a patch to use PKG_CHECK_MODULES instead, tested with gnutls-2.7.8 and gnutls-2.6.6, please review

Current output configure with new gnutls:
checking for libgnutls-config... no
checking for libgnutls - version >= 2.0.4... no
* The libgnutls-config script installed by LIBGNUTLS could not be found
* If LIBGNUTLS was installed in PREFIX, make sure PREFIX/bin is in
* your path, or set the LIBGNUTLS_CONFIG environment variable to the
* full path to libgnutls-config.
configure: error:
*
* libgnutls 2.0.4 or greater was not found. You may want to get it from
* ​ftp://ftp.gnutls.org/pub/gnutls/

Thanks

[Tim Kosse]

Problem originally reported at ​http://bugs.gentoo.org/show_bug.cgi?id=268583

[Tim Kosse]

Thanks.

I've applied a modified version that falls back to libgnutls-config if the check using pkg-config fails. I'm unable to compile pkg-config on mingw/msys.

[Bernard Cafarelli]

Even better! Thanks for quick commit