qt5webkit: select leveldb package and memenv

qt5webkit: select leveldb package and memenv

This patch fixes the build issue reported by autobuilder [0].

        /home/naourr/work/instance-2/output/build/qt5webkit-5.9.1/Source/WebCore//.obj/platform/leveldb/LevelDBDatabase.o: In function
	`WebCore::LevelDBDatabase::openInMemory(WebCore::LevelDBComparator const*)':
	LevelDBDatabase.cpp.text._ZN7WebCore15LevelDBDatabase12openInMemoryEPKNS_17LevelDBComparatorE+0x34): undefined reference to `leveldb::NewMemEnv(leveldb::Env*)'
        collect2: error: ld returned 1 exit status
        make[3]: *** [Makefile.api:97: ../lib/libQt5WebKit.so.5.9.1]
	Error 1

The issue happens when both packages leveldb and qt5webkit are enabled.

QtWebKit builds its own copy of leveldb [1] (as a third-party) if the
system does not provided it (i.e. buildroot). It builds it differently
and this is the origin of that issue. Instead of using the Makefile
provided by leveldb [2], QtWebKit uses qmake to build that library [3].

The missing symbol issue happens because the symbol leveldb::NewMemEnv
is bundled in the static library libmemenv.a (aside libleveldb.so).
This static library consists of this single symbol which is like an
extra that is built but *NOT* shipped by default at installation in the
staging directory. Unfortunatly, that symbol is required later by
WebCore [4].

The copy built by QtWebKit is an all-in-one library including both
libleveldb and libmemenv; thus QtWebKit links against libleveldb only.
Also, the linker finds the buildroot's copy first (not the third-party):
that explains why it is complaining about a missing symbol. That copy
does not have the symbol leveldb::NewMemEnv.

Fortunatly, QtWebKit provides a facility to link against the system
leveldb package. The qmake flag WEBKIT_CONFIG+=use_system_leveldb tells
Qt5WebKit to link against libleveldb *AND* libmemenv [5].

To fix that issue, this commit selects the package leveldb that now
installs the libmemenv static library and its header. It ensures that
QtWebKit has everything it needs to be built. It also sets the
appropriate qmake configure flags to tell QtWebKit to use the leveldb
copy built by buildroot instead of the bundled one.

[0]: http://autobuild.buildroot.net/results/46033e82adf592c3b92c6d50cfaf45bd58beeaa4
[1]: https://github.com/qt/qtwebkit/tree/5.9/Source/ThirdParty/leveldb
[2]: https://github.com/qt/qtwebkit/blob/5.9/Source/ThirdParty/leveldb/Makefile#L167-L169
[3]: https://github.com/qt/qtwebkit/blob/5.9/Source/ThirdParty/leveldb/Target.pri#L80
[4]: https://github.com/qt/qtwebkit/blob/5.9/Source/WebCore/platform/leveldb/LevelDBDatabase.cpp#L185
[5]: https://github.com/qt/qtwebkit/blob/5.9/Source/WebCore/WebCore.pri#L254
[6]: https://github.com/google/leveldb/commit/739c25100e46576cdcdfff2d6f43f9f7008103c7

Signed-off-by: Gaël PORTAY <gael.portay@collabora.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>