Matthew Woehlke authored Aug 15, 2016 and Brad King committed Aug 16, 2016

Add a new SOURCE_SUBDIR option to ExternalProject_Add that allows
specifying the location of the CMakeLists.txt to use as the project root
relative to the SOURCE_DIR.

This is helpful for projects that have unusual layouts, or projects that
provide both a superbuild and project-only build depending on which
CMakeLists.txt is used.

Fixes: #15118