SConstruct: validate prefix much less
SConstruct: validate prefix much less

Our porters sometimes wish to install with

  scons prefix=/opt/nonexistent --install-sandbox=/somewhere/else

however SCons's current handling of PathVariables makes this
impossible, and even validator=PathVariable.PathAccept doesn't work.

* Now accept nonexistent paths
* Now only require that the path is syntactically absolute

Drawback with this: if the prefix doesn't exist, users may reasonably
expect that it will be created. The docs will need to be updated to
tell users to do "scons prefix=/tmp/nonexistant install" instead.

Possibly addresses mypaint/mypaint#347.

b3sigma/libmypaint,achadwick/libmypaint,achadwick/libmypaint,achadwick/libmypaint,b3sigma/libmypaint,achadwick/libmypaint,b3sigma/libmypaint