[GTK] Child processes should be installed in a versioned directory
https://bugs.webkit.org/show_bug.cgi?id=135754

Reviewed by Gustavo Noronha Silva.

Define LIBEXEC_INSTALL_DIR as
${CMAKE_INSTALL_FULL_LIBEXECDIR}/webkitgtk-${WEBKITGTK_API_VERSION}
so that the child processes are installed in $libexec/webkitgtk-4.0.
This makes it possible to install 2.6 in parallel to older versions.

* Source/cmake/OptionsGTK.cmake:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@172376 268f45cc-cd09-0410-ab3c-d52691b4dbfc