SConstruct: Place toplevel include symlinks in core/build/include

This is friendlier than core/../build/include, which breaks if core is
built standalone.
Merge branch 'master' of github.com:otherlab/core
Ignore build/include
SConstruct: Fix PREFIX
Merge branch 'master' of github.com:otherfab/core
Merge branch 'master' of github.com:otherlab/core
Add back removed entries to .gitignore
Add toplevel include to point to othercore