Version details: 2.7.6
Distribution/Version: 5.3

The following modifications were necessary to compile gtk2 on IRIX5.3:
- it had to be configured using ac_cv_lib_X11_XkbQueryExtension=no.
- gtk/xdgmime/xdgmimecache.c needed the following extra lines:

ifndef MAP_FAILED
#define MAP_FAILED      ((void *) -1)
#endif
typedef unsigned short          uint16_t;
typedef unsigned int            uint32_t;

- gtk/gtkiconcache.c needed the following extra lines:

#ifndef MAP_FAILED
#define MAP_FAILED      ((void *) -1)
#endif

- On IRIX 5.3 /usr/include/X11/Xlib.h defines:

typedef struct _XIMStringConversionCallbackStruct {
    XIMStringConversionPosition position;
    XIMStringConversionType type;
    XIMStringConversionOperation operation;
    unsigned short factor;
    XIMStringConversionText *text;
} XIMStringConversionCallbackStruct;

please note the second item is called type, not direction as on other 
OSes, e.g. Linux.
So I had to make the following change to the code:

--- modules/input/gtkimcontextxim.c.orig        Sat Jun 18 23:50:32 2005
+++ modules/input/gtkimcontextxim.c     Sun Jun 19 00:37:17 2005
@@ -1245,7 +1245,7 @@
             return;
         }
 
-      switch (conv_data->direction)
+      switch (conv_data->type)
         {
         case XIMForwardChar:
           for (i = conv_data->factor, q = p; i > 0 && *q; --i)

I am not sure if this is accurate though.