Required for module test to pass.

When creating a library or executable, several source files are combined
into a binary artifact that has an outbound license of its own. This test
generator allows to check if the combined source files are compatible
with the desired outbound license.

Requirements for using these tests:
- input source files must contain the SPDX-License-Information tag
- python3 must be available
- the "reuse spdx" tool must be available

As CheckAtomic.cmake is a copy and we need to keep sync.

a348de480d34257ffe394ab51b880c9aef243d80
Date: 04/18/2020
repo: https://github.com/llvm/llvm-project
file: /llvm/cmake/modules/CheckAtomic.cmake

Application need to test, if they need to build against atomic libray.
In order to not duplicate the code over and over again, let's have one
version in ecm.

Our August releases produce invalid octal numbers ("08") otherwise.

qmlplugindump sometimes hangs forever and thus blocks a build. Setting an (arguably arbitrary) timeout works around that.