add `CFLAGS+=...` handling to `configure` #148

Job	Run time
ci (tpb, ubuntu-22.04, --pb --threads --enable-libffi, test-some-fast)	28m 3s
ci (a6osx, macos-12)	17m 55s
ci (ta6osx, macos-12)	23m 17s
ci (i3le, ubuntu-22.04)	11m 26s
ci (ti3le, ubuntu-22.04)	14m 35s
ci (a6le, ubuntu-22.04)	8m 53s
ci (ta6le, ubuntu-22.04)	12m 3s
ci (ta6le, ubuntu-22.04, reboot, -m=ta6le --force, re.boot SCHEME="chezscheme", test-some-fast)	8m 14s
ci (i3nt, windows-2022, vs)	11m 36s
ci (ti3nt, windows-2022, vs)	14m 37s
ci (a6nt, windows-2022, vs)	12m 52s
ci (ta6nt, windows-2022, vs)	15m 31s
ci (ta6nt, windows-2022, gcc)	13m 37s
	3h 12m 39s

Provide feedback