Static build fix - only attempt to link against libtasn1 if it's present

GnuTLS may be compiled without external libtasn1, so we shouldn't try
to link against it unless it's actually present.
Merge remote-tracking branch 'upstream/master' into buildfixes
Back out previous change

Needed to merge upstream changes in before creating pull request.
Static build fix - only attempt to link against libtasn1 if it's present

GnuTLS may be compiled without external libtasn1, so we shouldn't try
to link against it unless it's actually present.
Merge pull request #59 from bphinz/buildfixes

Static build fix - only attempt to link against libtasn1 if it's present