Use target_compile_definitions() for build flags

This change moves the bmcweb build flags out of settings.hpp
and into the CMakeLists.txt file as target_compile_definitions().

This makes it so it is no longer required to #include settings.hpp
to use build flags in the source.

Tested: Enabled and disabled some Redfish flags and confirmed that
the desired components were added and removed.

Change-Id: Ibeedb4fc8f3dcc286c73843823693a04c55c0615
Signed-off-by: Jason M. Bills <jason.m.bills@linux.intel.com>