On systems where cc != gcc, compilation likely fails because we are
using quite some gccisms in adu. So default to gcc but let the user
override the default by setting CC manually.
The straight-forward CC ?= gcc does not work with gnu make as make
assigns CC the default value "cc".
Thanks to Steffen Schmidt for pointing out this shortcoming.
LDFLAGS += -L/usr/local/lib
PREFIX ?= /usr/local
+ifeq "$(origin CC)" "default"
+ CC := gcc
+endif
ifeq (,$(findstring BSD,$(uname_S)))