Getting long SOURCES lines with subdirs shorter

[Jan Engelhardt]

Given

        a_SOURCES = aprog/main.c aprog/foo.c aprog/bar.c aprog/baz.c ...

The more source files there are to be listed, the longer that line gets, 
the bigger the Makefile.am fragment becomes, etc. I am thinking about 
how to cut that repetition down. Current automake likely won't have 
anything in store already, so I'm thinking of editing automake and 
targeting a future automake release.

The syntax would need to be understood by automake and expanded at the 
right time; Makefile.in should have the expanded list already. How 
about recognizing the gmake function syntax?

a_SOURCES = $(addprefix aprog/,main.c foo.c bar.c baz.c)