Linux MGCB fixes

Fixes compiling spritefonts and fbx models(tested with blender) under linux.

I will still need to update Pipeline.exe for 2 things:
1) Add libassimp.so to copy to output dir
2) Disable local copy for gtk libraries
I don't think this will work on a Mac
exe is MGCB.exe in this case (or it should be)  and under MacOS (and Linux?) it should be run as
mono MGCB.exe {args}

I used the following in my branch

https://github.com/dellis1972/MonoGame/blob/xsplugin/Tools/Pipeline/Gtk/Pipeline.Gtk/MainView.cs#L438