add DYNAMIC_LIB_EXT to Irix case

dtool/Config.Irix.pp

@@ -133,3 +133,6 @@
 // What is the syntax of the STL allocator declaration?  See
 // LocalSetup.pp for allowable values.
 #define STL_ALLOCATOR OLD
+
+// The dynamic library file extension (usually .so .dll or .dylib):
+#define DYNAMIC_LIB_EXT .so