cmake.bbclass: modify construction of compiler flags
Use CFLAGS instead of CPPFLAGS for C_FLAGS variants.
When debug optimization is enabled in the local.conf, the debug (-O0) vs
production (-O2) does not change in the builds. As the CPPFLAGS do not
contain the optimization settings.
Also the CXX_FLAGS are based on CXXFLAGS, so it makes sense to similarly
set the C_FLAGS based on CFLAGS.
Signed-off-by: Joe Slater <jslater@windriver.com> Signed-off-by: Jeff Polk <jeff.polk@windriver.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
