Remove libraries not required/used with GnuTLS

Based on Andreas Metzler's patch for libetpan Debian package:
http://anonscm.debian.org/gitweb/?p=users/mones/libetpan.git;a=blob;f=patches/10_unnecessary_linkage.diff;h=d1db5adb
Merge pull request #142 from mones/master

Remove libraries not required/used with GnuTLS