configure: when transfering ENABLE/OMIT flags from CFLAGS to OPT_FEATURE_FLAGS, also...

configure: when transfering ENABLE/OMIT flags from CFLAGS to OPT_FEATURE_FLAGS, also do the same for CPPFLAGS and remove those ENABLE/OMIT flags from CFLAGS/CPPFLAGS to mimic legacy build behavior. Strip ENABLE/OMIT flags from BUILD_CFLAGS but do not transfer those to OPT_FEATURE_FLAGS, also to mimic legacy behavior. This is the second part of a fix discussed at [forum:9801e54665afd728|forum post 9801e54665afd728].

FossilOrigin-Name: 16d307cc6c1e203900e7a2dc0730fc0e453946622a2114a07d64ebb99045cfbf