McCode: installation and location of special libraries such as GSL

[ebknudsen]

a few components need special libraries (e.g. GSL), that have to be linked simulation compile time. This is nicely handled by the DEPENDENCY-keyword, but if the libraries are not on the loadpath, we need extra "-I" and "-L" compiler directives. This is the case for windows where we ship the gsl-libraries with miniconda. At present there's a dummy bogus path "@MCCODE_LIB@/miniconda3/" set on these comps, which is overwritten by the windows build script to read "c:\mcstas-2.5\miniconda3\Library\mingw-w64". On the other systems the bogus path is simply ignored.

We should consider some simpler cleaner way if possible.