Running ./build.sh dist-war I get two errors:
1) BUILD FAILED $home/exist/build/scripts/dist.xml:35: $MYBUILDDIR/extensions/indexes/spatial/lib does not exist.
2) BUILD FAILED $home/exist/build/scripts/dist.xml:35: $MYBUILDDIR/extensions/betterform/main/xrx does not exist.

The problem is that the $home/exist/extensions/indexes/spatial/lib/ and $home/exist/extensions/betterform/main/xrx/ directories do not exist. Manually creating them before running dist-war stops the errors from recurring.