Wanted to add the SquirrelMail versioning functions we'd talked about before

Wanted to add the SquirrelMail versioning functions we'd talked about before
we split the streams to the new RC for 1.4.

check_sm_version performs similar function to check_php_version, and
will return true if version is >= what is specified.

For Thijs/Jonathan - the check_sm_version will return false if the
constant it checks against is not defined, making the function itself
safe for previous levels,  should you want to roll it back to stable
to allow plugin developers to use the check_sm_version function to
test for 'new' elements like SM_PATH, etc. that are present in 1.3.x
and up.

More info (though not a ton, I'll admit) is in the sm2-planning module
in cvs: cvs co sm2-planning, then pull up sm2-planning/index.php in your
browser.

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@4319 7612ce4b-ef26-0410-bec9-ea0150e637f0