cjson: link with libm, unavailable for static

Link with libm as stated in the README, fixes:
http://autobuild.buildroot.net/results/57f/57fc124e14263ee2447e20a5b910ed3ae0a5b7db/



Also disable the package for static builds since it doesn't work that
way.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>