cc-tool: fix static build problem with boost regex

The boost.m4 logic is a bit crappy, and needs some help to know that
boost_regex needs the pthread library.

Fixes:

  http://autobuild.buildroot.org/results/69a/69aff0b9d0ac8fe08e1d2f7ffb691f6a8fc8693b/



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