build: Make generating of HTML man work

AM_CONDITIONAL was defined too soon, so generating of HTML version of
man pages hasn't worked.

Solution is to move AM_CONDITIONAL to right place. Also because HTML
generating wasn't working, it is now disabled by default and one can
enable it by passing --with-html_man to configure script.

Signed-off-by: Jan Friesse <jfriesse@redhat.com>