log4cxx: keeping in-source build (resolves #252)

Adding 'B = ${S}' is preferred over using the autotools-brokensep
class. Using the autotools-brokensep is only backwards compatible
down to the openembedded-core commit 006b8a78 [1], whereas this
change is backwards compatible to even earlier versions.
This change works with openembedded-core commit f5bc3cfa [2], and
I expect it to work with even much more earlier versions.

[1] http://cgit.openembedded.org/openembedded-core/commit/?id=006b8a7808a58713af16c326dc37d07765334b12
[2] http://cgit.openembedded.org/openembedded-core/commit/?id=f5bc3cfac9545c402b415695c4e0f98ad38fb2b0