test_cmake_config_module fails for in-source builds

[junghans]

To reproduce:

cd cereal
cmake .
make
ctest

fails with:

38: Test command: /usr/bin/cmake "-P" "/tmp/cereal-1.3.2/unittests/cmake-config-module.cmake"
38: Test timeout computed to be: 10000000
38: -- Configuring done
38: -- Generating done
38: -- Build files have been written to: /tmp/cereal-1.3.2
38: Error: could not load cache
38: CMake Error at cmake-config-module.cmake:30 (message):
38:   Cereal cmake install-step failed

Found in https://bugzilla.redhat.com/show_bug.cgi?id=2137999