Raise GnuTLS requirements to 3.x

This allows us to simplify things by getting rid of some old
compatibility code. People should really be using current versions
of GnuTLS anyway to stay secure.
diff --git a/BUILDING.txt b/BUILDING.txt
index 0cb830b..67a4f08 100644
--- a/BUILDING.txt
+++ b/BUILDING.txt
@@ -12,7 +12,7 @@
 -- FLTK 1.3.3 or later
 
 -- If building TLS support:
-   * GnuTLS
+   * GnuTLS 3.x
    * See "Building TLS Support" below.
 
 -- If building native language support (NLS):