build: set c standard via CMAKE_C_FLAGS
build: set c standard via CMAKE_C_FLAGS

Otherwise -std=c99 is set for C++ files as well and g++ complains about
that.

Change-Id: I398e1b227fc07622ef10bbfca04e9cc44b146a4d
Signed-off-by: Christian Himpel <c73051e86a3a422d01005cf3832402ece92ac402@intel.com>

hoangt/processor-trace,01org/processor-trace,breadbaconic/processor-trace,andikleen/processor-trace,hoangt/processor-trace,pombreda/processor-trace,pombreda/processor-trace,andikleen/processor-trace,01org/processor-trace,shadown/processor-trace,01org/processor-trace,breadbaconic/processor-trace,drupalhunter-team/processor-trace,drupalhunter-team/processor-trace,shadown/processor-trace