[GTK] REGRESSION(r171598, CMake): Introspection files are installed at the wrong path
https://bugs.webkit.org/show_bug.cgi?id=135934

Reviewed by Gustavo Noronha Silva.

Since r171598 gir files are installed under $datadir/webkitgtk-4.0/gir-1.0/.
The gir files should installed in $datadir/gir-1.0/ together with all other
gir files. We don't need to install introspection files in a versioned
directory because their filenames already contain the binary version. But before
r171598, the files were only installed to the right directory if the
gobject-instrospection pkg-config file was in the same prefix than the one we
wanted to install, because the gir and typelibs directories were extracted from
the pkg-config file. We should define INTROSPECTION_INSTALL_GIRDIR and
INTROSPECTION_INSTALL_TYPELIBDIR in OptionsGTK.cmake based on $datadir and $libdir
like we do in the autotools build.

* Source/cmake/FindGObjectIntrospection.cmake: Do not define
INTROSPECTION_INSTALL_GIRDIR and INTROSPECTION_INSTALL_TYPELIBDIR.
* Source/cmake/OptionsGTK.cmake: Define
INTROSPECTION_INSTALL_GIRDIR and INTROSPECTION_INSTALL_TYPELIBDIR.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@172583 268f45cc-cd09-0410-ab3c-d52691b4dbfc