cd src0
scons
./main.out
# => No such file or directory.
../src1/main.out
# => 1
scons ..
./main.out
# => 0
../src1/main.out
# => 1
By default, SCons builds all targets under the current directory and descendants.
If Default() is given, it change that default to the given programs.
You can always go back to recurse under a given directory with scons path/to/dir.