Added cmake option to disable install rules

[ilya-lavrenov]

When pugixml is used as a submodule, installation rules are not required.

Additionally, it may cause issues when pugixml is installed / exported by parent project:

CMake Error: install(EXPORT "OpenVINODeveloperTargets" ...) includes target "inference_engine_s" which requires target "pugixml-static" that is not in this export set, but in multiple other export sets: runtime/cmake/OpenVINOTargets.cmake, lib/cmake/pugixml/pugixml-targets.cmake.
An exported target cannot depend upon another target which is exported multiple times. Consider consolidating the exports of the "pugixml-static" target to a single export.
CMake Error: install(EXPORT "OpenVINODeveloperTargets" ...) includes target "func_test_utils" which requires target "pugixml-static" that is not in this export set, but in multiple other export sets: runtime/cmake/OpenVINOTargets.cmake, lib/cmake/pugixml/pugixml-targets.cmake.
An exported target cannot depend upon another target which is exported multiple times. Consider consolidating the exports of the "pugixml-static" target to a single export.
CMake Error: install(EXPORT "OpenVINODeveloperTargets" ...) includes target "funcSharedTests" which requires target "pugixml-static" that is not in this export set, but in multiple other export sets: runtime/cmake/OpenVINOTargets.cmake, lib/cmake/pugixml/pugixml-targets.cmake.
An exported target cannot depend upon another target which is exported multiple times. Consider consolidating the exports of the "pugixml-static" target to a single export.

Parent projects now can set set(PUGIXML_INSTALL OFF) to disable installation rules.

It's a good practice to provide such option. Example is https://github.com/nlohmann/json/blob/develop/CMakeLists.txt#L47

[zeux]

Thanks! I'm open to merging this - it doesn't change the default behavior, and it seems fine to let people opt out of the installation rules. I'm not a CMake expert so I'm not sure why CMake can't figure things like this out on its own :)

Minor question - this PR changes the export statement to add pugixml-targets.cmake argument - is this intentional / how is this related?

[ilya-lavrenov]

@zeux before my change export command was used with export set:

export(EXPORT pugixml-targets
  NAMESPACE pugixml::)

From cmake install command:

install(TARGETS ${install-targets}
  EXPORT pugixml-targets
  ...

But now, since install rules are under PUGIXML_INSTALL condition, we have to change the used signature of export command - call with list of targets instead of export set

[zeux]

Thanks!