append to compile flags instead of overwriting

CFLAGS and LDFLAGS may be passed in by an upstream build system. we
want to make sure we absolutely add our dependency flags otherwise
stuff like hardening flags will break the build

Closes: #29

Makefile

@@ -10,14 +10,14 @@ INSTALLPATH = ${DESTDIR}${PREFIX}/bin
 MANPATH = ${DESTDIR}${PREFIX}/share/man/man1
 
 ifeq ($(shell sh -c 'which ncurses5-config>/dev/null 2>/dev/null && echo y'), y)
-	CFLAGS ?= -Wall -g $$(ncurses5-config --cflags)
-	LDFLAGS ?= $$(ncurses5-config --libs)
+	CFLAGS += -Wall -g $$(ncurses5-config --cflags)
+	LDFLAGS += $$(ncurses5-config --libs)
 else ifeq ($(shell sh -c 'which ncursesw5-config>/dev/null 2>/dev/null && echo y'), y)
-		CFLAGS ?= -Wall -g $$(ncursesw5-config --cflags)
-		LDFLAGS ?= $$(ncursesw5-config --libs)
+		CFLAGS += -Wall -g $$(ncursesw5-config --cflags)
+		LDFLAGS += $$(ncursesw5-config --libs)
 else
-	CFLAGS ?= -Wall -g $$(pkg-config --cflags ncurses)
-	LDFLAGS ?= $$(pkg-config --libs ncurses)
+	CFLAGS += -Wall -g $$(pkg-config --cflags ncurses)
+	LDFLAGS += $$(pkg-config --libs ncurses)
 endif
 
 tty-clock : ${SRC}