INSTALL: Update patch file links.

The ftp.idsoftware.com server died several months back. Some context:
<https://www.doomworld.com/forum/topic/96999-rip-ftpidsoftwarecom/>

Instead, point at the mirrors found in the idgames repository, using
the gamers.org version since this is the canonical copy everything else
mirrors from. Unfortunately it's not possible to point at the Doomworld
idgames interface since the patches don't appear in the database there.

man/INSTALL.template

@@ -22,9 +22,9 @@ to be installed:
 
  * A C compiler (gcc is recommended)
  * make (GNU make is recommended)
- * SDL2 (see http://www.libsdl.org/)
- * SDL2_mixer (see http://www.libsdl.org/projects/SDL_mixer/)
- * SDL2_net (see http://www.libsdl.org/projects/SDL_net/)
+ * SDL2 (see https://www.libsdl.org/)
+ * SDL2_mixer (see https://www.libsdl.org/projects/SDL_mixer/)
+ * SDL2_net (see https://www.libsdl.org/projects/SDL_net/)
  * Python (optional)
 
 Follow the standard instructions for installing an autotools-based
@@ -241,18 +241,18 @@ Upgrade patches are available that will update your game to the latest
 version, the following sites have the patches:
 
 #if DOOM
-  http://www.doom2.net/doom2/utils.html
-  ftp://ftp.idsoftware.com/idstuff/doom
-  ftp://ftp.idsoftware.com/idstuff/doom2
+  <http://www.doom2.net/doom2/utils.html>
+  <http://www.gamers.org/pub/idgames/idstuff/doom>
+  <http://www.gamers.org/pub/idgames/idstuff/doom2>
 #elif HERETIC
-  ftp://ftp.idsoftware.com/idstuff/heretic
+  <http://www.gamers.org/pub/idgames/idstuff/heretic>
 #elif HEXEN
-  ftp://ftp.idsoftware.com/idstuff/hexen
+  <http://www.gamers.org/pub/idgames/idstuff/hexen>
 #elif STRIFE
-  ftp://ftp.fu-berlin.de/pc/games/idgames/roguestuff
+  <http://www.gamers.org/pub/idgames/roguestuff>
 #endif
 
-Please see http://doomwiki.org/wiki/Game_patch for more information.
+Please see <http://doomwiki.org/wiki/Game_patch> for more information.
 
 #if _WIN32
 As the patches are binary patches that run as DOS executables, on