[GTK] Reorganize header files

https://bugs.webkit.org/show_bug.cgi?id=65616

Reviewed by Martin Robinson.

.:

* GNUmakefile.am: Initialize $libwebkitgtkincludedir to
$(prefix)/include/webkitgtk-<api-version>

Source/JavaScriptCore:

Install header files under $libwebkitgtkincludedir/JavaScriptCore.

* GNUmakefile.am: Use $libwebkitgtkincludedir.
* javascriptcoregtk.pc.in: Use webkitgtk-<api-version> as include dir.

Source/WebKit/gtk:

Install header files under $libwebkitgtkincludedir/webkit.

* GNUmakefile.am: Use $libwebkitgtkincludedir.
* webkit.pc.in: Use webkitgtk-<api-version> as include dir.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@96087 268f45cc-cd09-0410-ab3c-d52691b4dbfc