bug#74387: Cannot use Make conditionals

Hi Maxim - I think so much depends on the details of a given project
that I lack ideas for something to do in general.

One possibility that occurs to me in your case is to turn your automake
conditional into a make conditional, instead of mixing them.

Another possibility might be to move the make-specific stuff out of the
automake conditional and simplify away the +=, as in something like:

ifdef WITH_SLOW_TESTS
SLOW_TESTS = $(SH_TESTS_SLOW)
else
SLOW_TESTS = 
endif # WITH_SLOW_TESTS

if CAN_RUN_TESTS
..
TESTS = $(SH_TESTS) $(SLOW_TESTS)
endif CAN_RUN_TESTS

No real idea if it will avoid the other problems you had.

I guess I'm going to close this for lack of any way forward, but feel
free to reply/reopen or open a new issue if desired. --thanks, karl.