enable out of source builds
The changes should be GNU-Make / Autotools compatible. On the machines I have access to they enable out of source builds, for instance :
mculpo@nuvolari:~/epfl/swift$ tree -d -L 2
.
├── build
│ ├── doc
│ ├── examples
│ ├── src
│ └── tests
├── install
│ ├── bin
│ ├── include
│ └── lib
└── sources
├── autom4te.cache
├── doc
├── examples
├── m4
├── src
├── tests
└── theory
It enables having different build configurations pointing at the same source checkout.