The "openssl/opensslconf.h" header is copied from an
architecture-specific source file, which means that specific path is
not valid within the source tree. I previously hacked around this by
copying the file within the source tree, but that had the major
downside of invalidating various cache layers after the copy was
performed.
Since we install this header with the right name, a slightly better
solution, hopefully, is to add the header install path as an include
directory. In theory, this will not invalidate any caching and improve
the build process slightly.
I had a big thing written here about how including opensslconf.h broke
but then I found out I had accidentally committed an arch-specific
opensslconf in the source tree somehow in the other branches. Whoops.
Anyway, the build system copies this around now so it should work even
better (???).
