REGRESSION(CMake): Make it possible to build without introspection

REGRESSION(CMake): Make it possible to build without introspection
https://bugs.webkit.org/show_bug.cgi?id=138006

Reviewed by Philippe Normand.

.:

Add ENABLE_INTROSPECTION option.

* Source/PlatformGTK.cmake: Dot not add gir global target if
introspection is disabled.
* Source/cmake/OptionsGTK.cmake: Do not add gir individual targets if
introspection is disabled.

Source/JavaScriptCore:

Do not install introspection files when introspection is disabled.

* PlatformGTK.cmake:

Source/WebKit2:

Do not install introspection files when introspection is disabled.

* PlatformGTK.cmake:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@175450 268f45cc-cd09-0410-ab3c-d52691b4dbfc