Generate man pages and completions by default if possible

Make GMX_BUILD_HELP a tristate option to support three different use
cases:
 - OFF: never build anything (default for source distributions, where
   everything is already generated)
 - ON: always build everything as part of the ALL target, give an error
   if anything fails.
 - AUTO: try to build man pages and completions as part of the ALL
   target, and install the result if successful; if it fails, some
   warnings will be given, but build will still be otherwise functional.
This allows making the default AUTO for builds from a git repo, making
the shell completions and man pages work automatically also in such
cases (as long as the build host can run the compiled executables).

HTML help is generated only with GMX_BUILD_HELP=ON, since it is
currently in somewhat of a flux what will happen to it, and most people
will not notice it missing, either.

Change-Id: Id7a69b1b912381a2f1ad3678b82cbaa01cba8479