A more practical way is when make runs configure automatically:
1. cd <firstpath>/; ./configure
SRC_PATH=<firstpath>/ is written into config_host.mak
2. cd <secondpath>/; touch configure; make
make now runs <firstpath>/configure, so configure
assumes it's an out of tree build
When this happens configure overwrites parts of
the current tree with symlinks.
Make the test more robust: create a canary file under
the build tree, then look for it in the source tree.
If there - we know it's a source build!