Add support for the GNU/Hurd architecture

[As with GNU/kFreeBSD, it's treated as "Linux": all three use the GNU libc
and runtime linker, which is mostly what matters for ioquake3. -smcv]

Bug-Debian: http://bugs.debian.org/679330
Reviewed-by: Simon McVittie <smcv@debian.org>

Makefile

@@ -299,7 +299,7 @@ LIB=lib
 INSTALL=install
 MKDIR=mkdir
 
-ifneq (,$(findstring "$(PLATFORM)", "linux" "gnu_kfreebsd" "kfreebsd-gnu"))
+ifneq (,$(findstring "$(PLATFORM)", "linux" "gnu_kfreebsd" "kfreebsd-gnu" "gnu"))
 
   ifeq ($(ARCH),x86_64)
     LIB=lib64