Bug 452097 - Adapt to WebKitGTK2 so name change

The new so name is libwebkit2gtk-4.0.so.37 . Use only it if SWT_WEBKIT2
is set to 1 as it is still experimental, disabled by default and testing
with older so name is unlikely to happen.

Change-Id: Ib915dd7e548e23f7c909e4d8fbeaf60faa108af5
Signed-off-by: Alexander Kurtakov <akurtako@redhat.com>