The conditional variable assignment operator in Makefiles (`?=`) will
only assign a value if its not defined yet. However, CFLAGS/LDFLAGS are
commonly defined as environment variables to pass custom compiler/linker
flags (e.g. `-Os`). Unfortunately, Cyclone adds mandatory compiler flags
(without which it doesn't compile) via the conditional variable
assignment operator which is incorrect as these flags will not be added
if CFLAGS/LDFLAGS is defined in the environment. This commit fixes this
issue by appending flags to CFLAGS/LDFLAGS instead of using the
conditional assignment operator.
