build: remove an errant comma
build: fix add_definition abuse in CMake

add_definitions is meant for adding C preprocessor definitions.  Modern cmake
suggests use of the CMAKE_CXX_FLAGS for the purposes of pushing flags to the
compilation commands.  Simply switch to the modern form given that we are
already requiring a new enough cmake.
build: fix erroneous overwriting of flags

Always use list(APPEND) as it will perform the desired action even if the list
is empty or previously unset.  The first set is harmless, however, the
subsequent set was overwriting the previous flag setup resulting in an improper
compilation command being generated.  This manifested as a build failure on
Linux when using cmake + ninja.
build: remove unnecessary modification of CMAKE_REQUIRED_DEFINITIONS

This is unnecessary now that the flag handling has been fixed.  The flags will
be added properly in the main CMakeLists.txt after the config-ix inclusion which
performs the required check.