When repa is included in a parent project via FetchContent,
CMAKE_SOURCE_DIR and CMAKE_BINARY_DIR refer to the parent project,
while PROJECT_SOURCE_DIR and PROJECT_BINARY_DIR still refer to repa.

When repa is included in a parent project via FetchContent, the
parent project options are inherited in the repa project, which
can lead to name conflicts. For example, the parent project may
want to build and run its own tests but none of the repa tests.