Use variables for required library versions

We specify the versions of different libraries we need at different
places. First there's everything that libmx needs in configure.ac, and
then there's everything mxsh needs in libmx_mxsh.m4. Some of these are
common and we could have that specified in a common place such as
configure.ac.

Specify variables for the minimum required version of each library we
depend on, and use that variable where needed.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>