YCM-based superbuild does not work when cross-compiling with a toolchain that sets CMAKE_FIND_ROOT_PATH_MODE_PACKAGE to ONLY

[traversaro]

Some CMake toolchain for cross-compiling set the CMAKE_FIND_ROOT_PATH_MODE_PACKAGE CMake variable to ONLY . For an example of such toolchain, see Emscripten's one at : https://github.com/emscripten-core/emscripten/blob/1.38.30/cmake/Modules/Platform/Emscripten.cmake#L189 .

When this option is used with the ONLY option, the CMAKE_PREFIX_PATH variable, that is used by the superbuild's subproject and by the YCMBootstrap to find the superbuild subprojects already installed, is ignored by find_package calls, and it is only possible to specify directories in which to search by setting the CMAKE_FIND_ROOT_PATH CMake variable. A possible solution is indeed to check in the superbuild if CMAKE_FIND_ROOT_PATH_MODE_PACKAGE is set to ONLY and propagate/set CMAKE_FIND_ROOT_PATH instead of propagating and setting CMAKE_PREFIX_PATH.

cc @diegoferigo

[traversaro]

Related conan issue: conan-io/conan#4967 .