autotools: fix 'undefined reference' if libnfs was built with gnutls

[aeolio]

Linking against libnfs fails with 'undefined reference' to multiple gnutls_ symbols, if libnfs was built with gnutls and the build system uses pkg-config to determine the libraries that need to be linked to a target.

/home/buildroot/instance-0/output-1/host/opt/ext-toolchain/bin/../lib/gcc/sparc64-buildroot-linux-gnu/15.1.0/../../../../sparc64-buildroot-linux-gnu/bin/ld: /home/buildroot/instance-0/output-1/host/sparc64-buildroot-linux-gnu/sysroot/usr/lib/libnfs.so: undefined reference to `gnutls_certificate_set_x509_system_trust'
/home/buildroot/instance-0/output-1/host/opt/ext-toolchain/bin/../lib/gcc/sparc64-buildroot-linux-gnu/15.1.0/../../../../sparc64-buildroot-linux-gnu/bin/ld: /home/buildroot/instance-0/output-1/host/sparc64-buildroot-linux-gnu/sysroot/usr/lib/libnfs.so: undefined reference to `gnutls_set_default_priority'
/home/buildroot/instance-0/output-1/host/opt/ext-toolchain/bin/../lib/gcc/sparc64-buildroot-linux-gnu/15.1.0/../../../../sparc64-buildroot-linux-gnu/bin/ld: /home/buildroot/instance-0/output-1/host/sparc64-buildroot-linux-gnu/sysroot/usr/lib/libnfs.so: undefined reference to `gnutls_certificate_verification_status_print'
...

If TLS is detected by ./configure, add gnutls to the list of libraries in libnfs.pc.in.

Fixes: #586