FetchContent: Build fails if FETCHCONTENT_SOURCE_DIR_<UC_NAME>=<bad folder>
This seems like a bug, or at least an exception warranting a warning message.
If I already have an operating CMake build but then attempt a "developer override" by
cmake -G .. -S.. -B.. -DFETCHCONTENT_SOURCE_DIR_MYDEP=<bad folder>
the subsequent cmake --build and cmake --install will forget that MYDEP even exists (kinda' makes sense since FetchContent doesn't find a CMakeLists.txt for MYDEP). The
<bad folder> in this case is a path that simply doesn't exist.
The evidence for failure is that MYDEP fails to show up in the lines of
[ xx%] Built target XXX emitted by --build. More evidence is
--install .. --component mydep fails to emit anything beyond the 1st line
-- Install configuration:.
I'm using CMake 3.16 on Win10/Pro/64.