pipe: Add PIPE_OS_HURD

One tiny step toward porting Gallium to the GNU/Hurd kernel
(and fixing Debian bug #585618).

Signed-off-by: Corbin Simpson <MostAwesomeDude@gmail.com>
diff --git a/src/gallium/include/pipe/p_config.h b/src/gallium/include/pipe/p_config.h
index b81702a..c6ea198 100644
--- a/src/gallium/include/pipe/p_config.h
+++ b/src/gallium/include/pipe/p_config.h
@@ -146,6 +146,11 @@
 #define PIPE_OS_UNIX
 #endif
 
+#if defined(__GNU__)
+#define PIPE_OS_HURD
+#define PIPE_OS_UNIX
+#endif
+
 #if defined(__sun)
 #define PIPE_OS_SOLARIS
 #define PIPE_OS_UNIX