configure: Fix libnfs cflags and libs

configure: Fix libnfs cflags and libs

libnfs version 16 requires the gnutls library. Without specifying at
least -lgnutls, builds fail:

  LINK fio
/usr/bin/ld: /usr/lib/gcc/x86_64-redhat-linux/15/../../../../lib64/libnfs.so: undefined reference to `gnutls_certificate_set_x509_trust_dir'
/usr/bin/ld: /usr/lib/gcc/x86_64-redhat-linux/15/../../../../lib64/libnfs.so: undefined reference to `gnutls_transport_set_int2'
...

Modify the configure script to add cflags and library options for gnutls
to correctly build libnfs engine.

Also make sure that the CI install the gnutls library header files.

Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Link: https://lore.kernel.org/r/20250514070754.38281-1-dlemoal@kernel.org
Signed-off-by: Vincent Fu <vincent.fu@samsung.com>