Fix out-of-tree builds

It's not possible to build the project outside the source directory. It
will fail when it can't find libmx.la. This is due to a mistake in the
Makefile.ams where libmx.la is referred to in the source directory.
Since this file is generated in the build directory the reference should
go to the build directory as well.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>