Allow builds with disabled documentation

[German M. Bravo]

When using disabled documentation (--disable-documentation), building and installing (make install): xapian-core installs fine but xapian-bindings (I tried python) don't (missing docs/html).

Making distributions (make dist) with disabled documentation should work, but in the process there are many details which make it not: during the build, xapian-core compiles and produces a tar.xz file; the xapian-bindings doesn't... after some tweaking adding "fake" docs/html directories and index.html files, it does produce a tar.xz file; however, during the building from the produced tar.xz files, even the xapian-core ends with errors about missing files.

[Olly Betts]

We explicitly do NOT support make dist if you disable documentation - see xapian-core/docs/Makefile.am:

# Note: we don't want dist tarballs built with docs missing, so for simplicity
# this Makefile.am blindly assumes the tools are in place when "make dist" is
# being run.  For "make all", "make check", etc we carefully skip any docs
# which we don't have the tools to generate.

Most other operations (including make install) should work though - it sounds like a problem with the recent changes to use sphinx to build the python docs.

Can you attach a build log?

[Olly Betts]

[f155d11] fixes make, and also make install for most languages; [d2974fb] fixes make install for python and python3.