Retain source folder structure for CMake build #76

Job	Run time
preparation (ON) / cmake_preparation	1m 9s
preparation (OFF) / cmake_preparation	1m 2s
sc_p4est_tests (ON) / sc_p4est_cmake_tests	3m 0s
sc_p4est_tests (OFF) / sc_p4est_cmake_tests	3m 59s
t8code_linkage_tests (OFF, Release) / t8code_cmake_tests	7m 24s
t8code_linkage_tests (OFF, Debug) / t8code_cmake_tests	10m 44s
t8code_linkage_tests (ON, Release) / t8code_cmake_tests	8m 2s
t8code_linkage_tests (ON, Debug) / t8code_cmake_tests	13m 12s
t8code_tests (OFF, Debug) / t8code_cmake_tests	5m 6s
t8code_tests (ON, Debug) / t8code_cmake_tests	4m 50s
t8code_tests (OFF, Release) / t8code_cmake_tests	2m 44s
t8code_tests (ON, Release) / t8code_cmake_tests	3m 9s
	1h 4m 21s

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