'uname -m' returns IP<number> for MIPS CPUs on IRIX.

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@5541 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

configure

@@ -341,7 +341,7 @@ case "$cpu" in
   "Power Macintosh"|ppc|powerpc)
     cpu="powerpc"
   ;;
-  mips|mipsel)
+  mips|mipsel|IP*)
     cpu="mips"
   ;;
   sun4u|sparc64)