include_regular_expression: Fix propagation to subdirectories
Refactoring in commit 4efda261 accidentally broke propagation of the include regex to subdirectories. Refactoring in commit 0aa34de5 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 (closed)