CPM apparently doesn't propagate variables set with `PARENT_SCOPE`

[SidneyCogdill]

Testing locally with CPM uncovered this problem: apparently CPM prevents the variables set by subdirectory projects from propagating to parent project. On the other hand CPM automatically sets a proxy_SOURCE_DIR variable instead which can be used to locate the include directory.

Note this issue doesn't affect FetchContent_Declare.

The issue is addressed in microsoft/proxy@62f6c4d .

include(cmake/CPM.cmake)
CPMAddPackage(
  NAME proxy
  GIT_TAG 62f6c4d1370c76a2f6a00cb16babc0092beee019
  GIT_REPOSITORY https://github.com/SidneyCogdill/proxy.git
)

Since with CPM there's no downstream packager, the microsoft/proxy repository itself is the library vendor, I think it will be a good idea to add a CI test (at a later time), to make sure that consuming with CPM actually works.

Originally posted by @SidneyCogdill in microsoft/proxy#293 (comment)

---

This is how the variable is set: https://github.com/microsoft/proxy/blob/62f6c4d1370c76a2f6a00cb16babc0092beee019/CMakeLists.txt#L46

# Do not set the module target if proxy is consumed as a subdirectory.
if(PROJECT_IS_TOP_LEVEL)
  set(proxy_INCLUDE_DIR include)
  if(PROXY_BUILD_MODULES)
    include(cmake/proxyModuleTargets.cmake)
  endif()
else()
  # Propagate the variable to the parent project
  set(proxy_INCLUDE_DIR ${CMAKE_CURRENT_SOURCE_DIR}/include PARENT_SCOPE)
endif()