Easy debug build of libmuscle/libymmsl native

[LourensVeen]

We have a debug build of the native libraries, which I think is used by the tests. If you're just installing as normal though, then it seems that you get a libymmsl_d.so but not a debug version of libmuscle. That makes no sense. So far, few people seem to need a debug version so maybe it shouldn't be installed by default, but it should be easy to build and install in case it's needed, and at any rate we shouldn't have a libymmsl_d.so but not a libmuscle_d.so. To be investigated and cleaned up.