The gen-manpages.py script is unable to find
the executables because it tries to search in
/bitcoin/src when no custom BUILDDIR is set.
With the new build process in place, the default
build dir is now set to /bitcoin/build.
Prior to this change, the script stopped with a
not found or not an executable error. Now it
generates the man pages without needing to
explicitly set BUILDDIR variable.
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