[CMake] Define SHOULD_INSTALL_JS_SHELL before including ports Options files.

[CMake] Define SHOULD_INSTALL_JS_SHELL before including ports Options files.
https://bugs.webkit.org/show_bug.cgi?id=132525

Reviewed by Martin Robinson.

* CMakeLists.txt: If OPTION() is called after Options${PORT}.cmake is included, it will
override whatever value a port may have set for it. The GTK+ port, for example, tries to set
it to SHOULD_INSTALL_JS_SHELL to ON by default, even though it did not happen before.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@168225 268f45cc-cd09-0410-ab3c-d52691b4dbfc