Fix dcopc headers location

Thanks to Francois Andriot for the patch

dcopc/Makefile.am

@@ -9,8 +9,8 @@ libdcopc_la_SOURCES = dcopc.c marshal.c dcopobject.c
 libdcopc_la_LIBADD = -lSM -lICE $(GTK2_LIBS) $(GLIB2_LIBS)
 libdcopc_la_LDFLAGS = -version-info 1:0 $(X_LDFLAGS) -lICE -no-undefined
 
-include_HEADERS = dcopc.h marshal.h dcopobject.h util.h
-includedir = $(prefix)/include/dcopc
+dcopcinclude_HEADERS = dcopc.h marshal.h dcopobject.h util.h
+dcopcincludedir = $(includedir)/dcopc
 
 noinst_HEADERS = global.h