Hunter instances wait for each other

[NeroBurner]

• I've read Brief overview section and do understand basic concepts. [Yes]
• I've read F.A.Q. section and there is no solution to my problem there. [Yes]
• I've read Code of Conduct, I promise to be polite and will do my best at being constructive. [Yes]
• I've read Reporting bugs section carefully. [Yes]
• I'm using latest Hunter URL/SHA1. [No]

I'm using a shared HUNTER_ROOT directory for concurrent CI builds. The configurations have the same hunter-version, but different compiler or compiler flags therefore a different generated hunter-toolchain file and sha1. They should be able to work concurrently, but only one of the concurrent cmake projects is actually compiling a hunter package, all other instances are waiting for the former

The build folder should be locked per toolchain, not per hunter-version

[PyrO70]

There is start of explaining what locking is happening in this bug old issue 1806. I believe the download directory is lock when the downloading happens, but is not release until the build is finish. The build itself is lock for each toolchains.

The problem is that download step and build step is fused in ExternalProject_Add:

    https://github.com/ruslo/hunter/blob/v0.23.151/cmake/schemes/url_sha1_cmake.cmake.in#L142-L163

And for optimization reason Download directory can be shared between several CMake instances (and hence should be locked before modification).