Although this is not as rigorously correct as the previous arrangement
(introduced with 80834ea), i.e., from the perspective of
$(abs_top_builddir)/tests/Makefile (generated from from
$(top_srcdir)/tests/Makefile.am), for which $(top_builddir)
is literally "..":
- qbconfig.h is generated from $(top_srcdir)/include/qb/qbconfig.h.in
into $(top_builddir)/include/qb directory, and consumed from here
- rest of these header files are consumed directly from
$(top_srcdir)/include/qb directory
this new simplified variant:
- avoids code duplication (DRY)
- works thanks to VPATH in $(abs_top_builddir)/tests/Makefile
pointing to $(top_srcdir)/tests, which means that unsatisfied
dependencies on "$(top_builddir)/include/qb/%.h" (resolved to
"../include/qb/%.h") will be subsequently sought as
"$(top_srcdir)/tests/../include/qb/%.h", which has the same effect
as previous explicit "$(top_srcdir)/include/qb/%.h" specification
(note that such automatic VPATH provision is supported by automake
"since beginning" so no backward incompatibility is expected)
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