Implement VPATH builds

[elfring]

Describe the bug I would like to build another current version of this software also for my needs. Unfortunately, something hindered me.

Steps To Reproduce

1. Current contents were retrieved from the development repository.
2. Command examples

   Markus_Elfring@Sonne:…/Projekte/Bau/BOINC> /…/Projekte/BOINC/lokal/configure --enable-optimize --disable-server && LANG=C make
   …
   --- Configuring BOINC 7.23.0 (Release) ---
   --- Build Components: (client manager libraries libraries_graphics manager_clientscr) ---
   …
   Making all in api
   make[2]: Entering directory '…/Projekte/Bau/BOINC/api'
   make[2]: *** No rule to make target '…/Projekte/BOINC/lokal/api/boinc_api.cpp', needed by 'boinc_api.lo'.  Stop.
   …
   

Expected behavior The make command should succeed again as it happened before.

[AenBleidd]

Can't reproduce this on Ubuntu 20.04 with the current master 15d0db134c573787abd941b29605527470267edd Next commands were successfully executed:

./_autosetup
configure --enable-optimize --disable-server
LANG=C make

@elfring, please provide more information about your setup.

[elfring]

How do you think about to benefit from parallel build trees also for this software? Would you like to support out-of-source (VPATH) builds?

[elfring]

Another build attempt within the source directory succeeded also on my Linux system configuration.

:crystal_ball: How can the situation be improved for the support of other build directories?

[AenBleidd]

VPATH builds is an interesting feature. However, our current build system is stable for years, so this feature is not mandatory. @elfring, if you want to implement it - you are very welcome.

[AenBleidd]

I don't think it makes sense to invest time in this.