Makefile: prefer distro cflags if specified

The compilation flags that are necessary for building and part of the
developer's intentions belong in the `+=` section, but the base flags
that distros like to override belong in a `?=` section, so that build
systems using their own are respected.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

src/Makefile

@@ -12,7 +12,8 @@ VERSION := $(shell \
 DEPDIR := .deps
 DEPFLAGS = -MT $@ -MMD -MP -MF $(DEPDIR)/$*.d
 
-CFLAGS += -Wall -Werror -ggdb -fno-omit-frame-pointer -O2 -D_FORTIFY_SOURCE=2
+CFLAGS ?= -ggdb -fno-omit-frame-pointer -O2
+CFLAGS += -Wall -Werror -D_FORTIFY_SOURCE=2
 CFLAGS += -DVERSION=\"$(VERSION)\"
 CFLAGS += -Wno-error=deprecated-declarations