Fix the build with slibtool.

[orbea]

When building muffin with slibtool (https://dev.midipix.org/cross/slibtool) it fails, this PR has two commits to fix the build.

1. Several libtool .la files are missing as dependencies in some of the Makefile.am files.
2. The build fails with -no-undefined, this works with GNU libtool because it silently ignores -no-undefined while slibtool does not. This requires building src/compositor/plugins separately from the rest of src/ to ensure libmuffin.la exists when it is needed.

Full logs:

muffin-slibtool.1.log
muffin-slibtool.2.log

[mtwebster]

Thanks