Fixes problems with cmake detection of GnuTLS. The current CMakeLists.txt uses check...

Fixes problems with cmake detection of GnuTLS.  The current CMakeLists.txt uses check_function_exists to identify legacy versions of GnuTLS but cmake performs this test by linking a small test program.  If libgnutls, libgcrypt, or libgpg-error are outside the default library search path, linking the test program fails even though gnutls and it's dependencies are installed.  This patch makes it possible to specify the location of each of the three libraries independently and only as needed.

git-svn-id: svn://svn.code.sf.net/p/tigervnc/code/trunk@5160 3789f03b-4d11-0410-bbf8-ca57d06f2519