Fix links (mostly http -> https).
diff --git a/docs/INSTALL.GNU b/docs/INSTALL.GNU
index e314ecf..b15e0b5 100644
--- a/docs/INSTALL.GNU
+++ b/docs/INSTALL.GNU
@@ -23,7 +23,7 @@
     Note that  make++, a  make tool written  in Perl,  supports enough
     features of GNU make to compile FreeType.  See
 
-      http://makepp.sourceforge.net
+      https://makepp.sourceforge.net
 
     for more information;  you need version 2.0 or newer, and you must
     pass option `--norc-substitution'.