I have taken Daniel Espinosa's patch and improved it so that it fixes all the test failures. Some of the failures were actually left-over changes to the tests required by Daniel's patch, which (correctly) considers the primary location of C files generated by valac to be builddir, not srcdir.
One change in particular is quite awkward, and could do with other eyes, to see if it's correct, and even if so, whether it could be simplified: the additional check in line 5737 of
automake.in which checks whether there's a pre-built .c file in srcdir, and, if srcdir != builddir, copies the file there if it is newer than the corresponding .vala file.
Having worked on this patch, I do wonder whether the hacks involved are worth it. It seems to me there are two sensible options:
1. Add more support to automake for files that can be either generated or distributed. (Have I overlooked support for this?)
2. Remove the support for distributing Vala projects as C files that do not require a Vala compiler. The Vala compiler is portable and has much the same dependencies as most Vala projects (that is, a C compiler and glib).
Without one of these, bugs and fragility in Vala support are likely to continue.
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0001-Improve-Vala-support-closes-13002.patch