[CMake] Do build after GIT_TAG changed in ExternalProject_Add

[typ1232]

Thank you very much for your detailed response.

I updated to CMake version 3.2.2 and I'm also using VS 2013 Update 4.

When testing with your project, I noticed the first difference with your
description is here:

"Now if you run build.cmd after that, you should see the repo cloned 
the first time you run it. After that, unless you change the 
CMakeLists file, each build should be a "do nothing" build. "

For me the external project is additionally configured and built after it
was cloned and it will produce an output file
(build\name-prefix\src\name-build\Release\echoargs.exe).

Now when changing the GIT_TAG I get exactly what you describe. Only the
update step is executed to checkout the other commit.

But here I would expect that the project is built again, because the commit
has changed an input file (intentional_compile_warning.cxx). See:
https://github.com/dlrdave/SmallAndFast/commit/68c3cebf842aa8a13d659fec428fd85ca3a24307

Am I wrong with this intuitive expectation? To me it seems to be the logical
thing to do. If I change the GIT_TAG, I want to use the outputs of that
exact version and not some outdated one.

Our external project is a library to which we link. If it is not rebuilt
after GIT_TAG is changed, then there will be build errors.



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