cmake: Don't specify a C standard when building Kconfig

Specifying a C standard triggered a compiler warning on Ubuntu (gcc
5.4.0) and a compiler error on Mac OS 10.12.6. Omit specifying the
standard and let the host toolchain use it's default instead. Tested
on Mac OS and Ubuntu 16.04.3.

This fixes #5640

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>