Allow customizing the main targets to build (& delete)

[xparq]

This is to ease updating the Makefile in client projects -- which is still a manual process of copy-pasting portions of the Makefile! (-> also: #33)

Adding them by hand is pretty much trivial, but clean would know nothing about it, and leave them around.

(Doing the same for extra libs is far less of a pressing issue.)

Something like renaming finish: ... and delegating it to the prj layout config, so e.g. sfw would have it like

finish: $(main_lib) $(main_exe) $(test_exe)

Well, OK, using the target directly in the cfg. section is impossible, as all the macros that are typically used in those names would then get prematurely expanded (with incorrect/empty values)! So, another macro would be needed for that, to list the targets separately... :-/

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• main_lib/main_exe can just be not set (commented out)
• extra targets can also be listed in a new build = ... macro now
• also delete those in clean!