Makefile: Fix in-tree builds when Sphinx is available

In commit 27a296fc we switched the qemu-ga manpage over to
being built from Sphinx.  The makefile rules for this were correct
for an out-of-tree build, but break for in-tree builds if Sphinx is
present and we're trying to build the documentation.

Specifically, because Sphinx refuses to build output files into
the same directory as its sources, for an in-tree build we tell
it to build into a subdirectory docs/built, and set up a makefile
variable MANUAL_BUILDDIR indicating where the docs are going.
The makefile rule telling Make how to build qemu-ga.8 correctly
used this variable, but the lines adding qemu-ga.8 to the list
of DOCS to be built and the 'make install' rune did not. The
effect was that for an in-tree build we told Make to build
'docs/interop/qemu-ga.8' but did not provide a specific rule for
doing so, which caused Make to fall back to the old rules.make
rule for building any "%.8" file. Make tried to invoke texi2pod
with a bogus command line, resulting in the error:

  GEN     docs/interop/qemu-ga.8
No filename or title
make: *** [rules.mak:394: docs/interop/qemu-ga.8]

Fix this by using $(MANUAL_BUILDDIR) when constructing the
list of DOCS files we want to build and also in the source
file name we install for 'make install'.

(Among other things, this broke the Shippable CI builds.)

Fixes: 27a296fc
Reported-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20190919155957.12618-1-peter.maydell@linaro.org
Tested-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>