boost: do not allow boost-log on PowerPC with uClibc

The build of boost-log with a uClibc toolchain on PowerPC fails with
some weird C++ issues. Since nobody ever looked into this problem,
let's "fix" the autobuilder issue by not allowing to select boost-log
in such a situation.

Fixes:

   http://autobuild.buildroot.net/results/13b43105caf4d3952de70030b51f8d96cf6604ee/


   (and many, many similar build failures)

[Peter: also add dependency to comment]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>