Add support for Apple framework builds

[Treata11]

Add CMake option PUGIXML_BUILD_APPLE_FRAMEWORKS to build PugiXML as Apple Frameworks.
This is needed to be able to create iOS-derived compatible app bundles which need to embed PugiXML as framework, as using dylibs is prohibited by Apple.

CMake officially made support for framework bundles in v3.14:
https://cmake.org/cmake/help/latest/prop_tgt/FRAMEWORK.html#prop_tgt:FRAMEWORK

Configure (iOS):

cmake -S. -B build -DPUGIXML_BUILD_APPLE_FRAMEWORKS=TRUE -DPUGIXML_INSTALL=TRUE -DCMAKE_SYSTEM_NAME=iOS

Build:

sudo cmake --build build --config Release --target install

Built frameworks would be written at: build/pugixml.framework

[zeux]

Thanks!