The targets added by commit v3.6.0-rc1~240^2~2 (Ninja: Add `$subdir/all`
targets, 2016-03-11) use as `$subdir` the relative path from the top of
the source tree to the current source directory. This is not correct
when using `add_subdirectory(test test_bin)`. Instead we need to use
the relative path from the top of the binary tree to the current binary
directory as was done for related targets by commit v3.7.0-rc1~268^2
(Ninja: Add `$subdir/{test,install,package}` targets, 2016-08-05).
Alexis Murzeau