iconv message loader won't build when using current automake

The existing Makefile.in in src/xerces/util/MsgLoader/MsgCatalog` uses `$(mkdir_p). However, with current (and not so current) automake versions this is set to $(MKDIR_P), but this variable is unset, leading to a failed build:

gmake[4]: ../../../../../src/.libs: Command not found

(from build)

The attached patch substitutes the missing variable.