fix breakage when user overrides CFLAGS on the make command line

fix breakage when user overrides CFLAGS on the make command line

these per-target CFLAGS adjustments are mandatory additions to the
command line for building the affected targets, not part of the
user-provided CFLAGS for tuning. my intent was always that the
variable append operations would take place after user settings, but
when a variable is set on the command line, it overrides all
definitions in the makefile, including target-specific ones.

based on patch by Szabolcs Nagy.