Refactoring in commit v3.4.0-rc1~321^2 (cmMakefile: Remove special
handling of INCLUDE_REGULAR_EXPRESSION, 2015-04-04) accidentally broke
propagation of the include regex to subdirectories.  Refactoring in
commit v3.5.0-rc1~319^2~1 (cmState: Initialize properties immediately,
2015-10-07) moved maintenance of this value from `cmMakefile` to
`cmStateSnapshot`.

Restore propagation of the `INCLUDE_REGULAR_EXPRESSION` to
subdirectories and add a test to cover it.

Fixes: #17676