configure.ac: simplify search for docbook2X, add db2x_docbook2man

AC_PATH_PROGS can be used to search for the first usable program from a
given list.

In Fedora the correct docbook tool for creating manpages is
'db2x_docbook2man' from the docbook2X package.
Merge pull request #100 from michich/for-upstream

configure.ac: simplify search for docbook2X, add db2x_docbook2man