scons: Format output for readability

Add separators between configure and build phases, prepend
failed prerequisite checks with ERROR: suffix and reformat
code a little.
scons: Replace ERROR: prefix with WARN:

As noted by @loonycyborg in pr#161 failed Configure tests
are acceptable for some kind of builds.
Merge pull request #161 from techtonik/master

scons: Format output for readability