Uniform behavior for 'make man' and 'make html'

Both man and HTML pages are now generated and installed using similar
installation rules.  GMX_BUILD_HELP controls automatic generation of
both.  They are also put into the source distribution using the same
mechanism for both.

Part of #685 and #1242.

Change-Id: Id61e75623861b46f765da49c4b34047a6b327083