cppcms: needs NPTL support

cppcms: needs NPTL support

cppcms uses some thread functions that are not available in certain
uClibc thread implementations. Even though a bit more restrictive than
necessary, adding a NPTL dependency is the easiest solution, and is
quite logical for a relatively large and complex package such as
cppcms.

Fixes:

  http://autobuild.buildroot.net/results/a26/a26574419aacbea4140dfca1d503bcab599edd71/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>