configure: prepare CFLAGS/CXXFLAGS/LDFLAGS for Meson

Split between CFLAGS/QEMU_CFLAGS and CXXFLAGS/QEMU_CXXFLAGS so that
we will use CFLAGS and CXXFLAGS for flags that we do not want to
pass to add_project_arguments.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>