GNU/Hurd fixes

Here is a couple of fixes for GNU/Hurd:
- dri_interface.h: no libdrm support either.
- configure.ac:
 - GNU/Hurd is a GNU OS with _GNU_SOURCE and PTHREADS.
 - GNU needs a couple of flags like other OSes

Signed-off-by: Dan Nicholson <dbn.lists@gmail.com>
diff --git a/configure.ac b/configure.ac
index 8412cdc..7b07f0f 100644
--- a/configure.ac
+++ b/configure.ac
@@ -87,7 +87,7 @@
 DEFINES=""
 AC_SUBST([DEFINES])
 case "$host_os" in
-linux*|*-gnu*)
+linux*|*-gnu*|gnu*)
     DEFINES="$DEFINES -D_GNU_SOURCE -DPTHREADS"
     ;;
 solaris*)
@@ -742,6 +742,10 @@
                 unichrome savage sis swrast"
         fi
         ;;
+    gnu*)
+        DEFINES="$DEFINES -DUSE_EXTERNAL_DXTN_LIB=1 -DIN_DRI_DRIVER"
+        DEFINES="$DEFINES -DGLX_INDIRECT_RENDERING -DHAVE_ALIAS"
+	;;
     solaris*)
         DEFINES="$DEFINES -DUSE_EXTERNAL_DXTN_LIB=1 -DIN_DRI_DRIVER"
         DEFINES="$DEFINES -DGLX_INDIRECT_RENDERING"