[GTK] Move libLevelDB.la setup into a separate GNUmakefile.am

[GTK] Move libLevelDB.la setup into a separate GNUmakefile.am
https://bugs.webkit.org/show_bug.cgi?id=112947

Reviewed by Martin Robinson.

.:

* GNUmakefile.am: Include the new GNUmakefile.am, located under Source/ThirdParty/leveldb.

Source/ThirdParty:

* leveldb/GNUmakefile.am: Added, sets up the libLevelDB.la library.

Source/WebCore:

No new tests - no new functionality.

* GNUmakefile.am: Move the libLevelDD.la setup into the new GNUmakefile.am that's placed in
the third-party leveldb source directory.
* GNUmakefile.list.am: Move the LevelDB sources listing there as well.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@146498 268f45cc-cd09-0410-ab3c-d52691b4dbfc