Avoid recording -fdebug-prefix-map= from CFLAGS and CPPFLAGS
It's not particularly useful to record -fdebug-prefix-map= and friends in the list of used CFLAGS arguments. Additionally, since these compiler arguments are typically used to make reproducible binary artifacts by stripping out information about the ephemeral build path used, embedding them in the resulting binary actually re-introduces the reproducibility problem.
Note that gcc itself omits these values from the DWARF information that it generates:
You can see the reproducibility issue here:
https://tests.reproducible-builds.org/debian/dbd/unstable/amd64/knot_2.7.6-2.diffoscope.html