Cygwin: Conditionally build documentation

Cygwin: Conditionally build documentation

Add a configure option '--disable-doc' to disable building of the
documentation by the 'all' target.

Check for the required tools at configure time, and require them if
building documentation is enabled.

Even if building the documentation was diabled with '--disable-doc',
'make -C doc' at the top-level can still make the documentation, if the
documentation tools were found. If the tools were not found, 'missing'
is used to issue a warning about that.

Update instructions for building Cygwin appropriately.

(Building documentation remains the default to increase the chances of
noticing when building the documentation is broken.)