Let gmtmex be a regular GMT supplement

**Description of the desired feature**

Unlike PyGMT and GMT.jl, the GMT/MEX toolbox requires compiled interface code. We currently have a separate repo with a poorly controlled Xcode debug framework hacked form the GMT setup. Debugging gmt/mex from Matlab *.m files requires a bit of gymnastics in taht we must supply the GMT library as dependency to the gmtmex build but the run that gmtmex build in Xcode, rather than the GMT C lib which we do for PyGMT and GMT.jl. It thus makes sense to keep gmtmex under GMT so that it can simply inherit all the well-constrained CMake setup.

I expect we handle this by adding a parameter or two to the ConfigUserAdvancedTemplate.cmake setup since we will need to find the MATLAB mex libraries and includes during build. I will therefore make a branch to test this out and I may have questions since some configuring is only conditionally needed depending on GMT_MEX is true of similar.