Configuring with builtin xrootd can fail because of seemingly not found OpenSSL library

[pcanal]

Doing in a empty build directory

cmake -Dminimal=ON $ROOT_SRC_DIR
cmake -Dbuiltin_xrootd=ON .

fails with:

CMake Error at cmake/modules/SearchInstalledSoftware.cmake:924 (target_link_libraries):
  The link interface of target "XRootD" contains:

    OpenSSL::SSL

  but the target was not found.  Possible reasons include:

    * There is a typo in the target name.
    * A find_package call is missing for an IMPORTED target.
    * An ALIAS target is missing.

Call Stack (most recent call first):
  CMakeLists.txt:305 (include)

while on the same machine and in another empty directory doing just:

cmake -Dbuiltin_xrootd=ON .

The difference between the two case is that the ssl variable usually default to ON but in the minimal case it default to OFF and turning on builtin_xrootd does not turn ssl back on.

The reason why this matters is that find_package defaults to declaring its results only inside the parent_scope. In practice we see find_package works properly in the top level CMakeList.txt and in one level of indirection (typically cmake/modules/SearchInstalledSoftware.cmake) but a second level of direction (i.e. inside the add_subdirectory(builtins/xrootd) inside SearchInstalledSoftware.cmake), it does not work.

From https://cmake.org/cmake/help/latest/command/find_package.html we read:

New in version 3.24: Specifying the GLOBAL keyword will promote all imported targets to a global scope in the importing project. Alternatively, this functionality can be enabled by setting the CMAKE_FIND_PACKAGE_TARGETS_GLOBAL variable.

So there is 3 possible solution

1. Require CMake version 3.24 and add the GLOBAL keyword where needed (or set the variable)
2. Make sure that builtin_xrootd (and any other similar components) to turn ssl ON when turned ON
3. Move the find_package(OpenSSL) from inside builtins/xrootd/CMakeLists.txtto it inclusion point inSearchInstalledSoftware.cmake`
4. Add an error if builtin_xrootd is ON while ssl is OFF.

[guitargeek]

3. Move the find_package(OpenSSL) from inside builtins/xrootd/CMakeLists.txtto it inclusion point inSearchInstalledSoftware.cmake`

Sounds like a good and simple solution to me!