Aaron Orenstein authored Oct 26, 2017 and Brad King committed Nov 17, 2017

This reintroduces the change from commit v3.10.0-rc1~69^2 (Performance:
Improve efficiency of source file lookup in cmMakefile, 2017-08-17) with
some corrections.  The original was rolled back by commit
v3.10.0-rc1~52^2~1 (Revert "Performance: ...", 2017-09-25) due to
incompatibilities found.  The rollback was followed-up by addition of a
test for the offending case, and this revision passes the test.