CMake find_package needs to be tested in CI

[zeux]

v1.11 required three patches to introduce an alias for renamed pugixml::pugixml target; the alias was present in CMakeLists.txt but that doesn't propagate to the installed cmake config. The first patch worked on 3.18; the second patch worked on earlier versions, but not early enough - wasn't compatible with 3.10. The third patch fixed the problem.

We need a way to test this in CI. I was testing these locally with

cmake_minimum_required(VERSION 3.0)
project(test)
add_executable(pugitest test.cpp)
find_package(pugixml)
target_link_libraries(pugitest pugixml)

I believe the CI infra - between Travis and Appveyor - probably has a wide enough gamut of CMake versions for us to validate all of this. If someone could contribute the necessary changes that'd be fantastic...

[zeux]

List of CMake issues that would need to be covered by this test:

#383
#389
#390
#393