Don't overwrite ECM_GLOBAL_FIND_VERSION if already set
This fixes the scenario outlined by Friedrich in !144 (merged):
find_package(ECM 5.84)
(now ECM_GLOBAL_FIND_VERSION is at 5.84)
find_package(Qt 5.15)
find_package(Foo 1.2) # uses find_package(ECM 5.26) in chain pulled by FooConfig.cmake
(now ECM_GLOBAL_FIND_VERSION is at 5.26)
include(KDEInstallDirs)
include(KDECMakeSettings) # surprise, will auto-decide based on version decided not by us, but Foo
include(KDECompilerSettings NO_POLICY_SCOPE) # surprise, will auto-decide based on version decided not by us, but Foo
Edited by David Faure