add cmake variable RAPIDJSON_INSTALL to control installations

[ouonline]

Users can disable installations by setting RAPIDJSON_INSTALL to OFF

[dinomight]

What's the purpose of disabling installations? My understanding of CMake says these don't actually do anything unless you run CMake install. It also seems against best practices to invoke return() in the middle of a CMakeLists.txt and prone to causing issues in the future if content is added after the install definitions. It would be useful if your commit message went into a bit more detail in the reason for the change and how it's beneficial.

[ouonline]

What's the purpose of disabling installations? My understanding of CMake says these don't actually do anything unless you run CMake install. It also seems against best practices to invoke return() in the middle of a CMakeLists.txt and prone to causing issues in the future if content is added after the install definitions. It would be useful if your commit message went into a bit more detail in the reason for the change and how it's beneficial.

Disabling installations may be useful when a project uses Rapidjson internally and doesn't want export(or expose) Rapidjson symbols to users who use this project.

Putting return() in the middle of a CMakeLists.txt causes less diff contents but it is not a good practice here. I will push another patch for that.