Fix Makefile

- Respect system CC, CFLAGS, LDFLAGS if these are set
- Respect DESTDIR during installation

Makefile

@@ -1,29 +1,29 @@
 PROJECT = sdwmbar
-CC = cc
+CC ?= cc
 
-PREFIX = /usr/local
-XPREFIX = /usr/local
+PREFIX ?= /usr/local
+XPREFIX ?= /usr/local
 
 OS != uname
 .if ${OS} == OpenBSD
-XPREFIX = /usr/X11R6
+XPREFIX ?= /usr/X11R6
 .endif
 
 INCS   = -I${XPREFIX}/include
 LINKS  = -L${XPREFIX}/lib -lX11
-CFLAGS = -std=c99 -pedantic -Wall -Werror ${INCS} ${LINKS}
+CFLAGS += -std=c99 -pedantic -Wall -Werror ${INCS} ${LINKS}
 
 all:
-	cc ${PROJECT}.c ${CFLAGS} -o ${PROJECT}
+	${CC} ${PROJECT}.c ${CFLAGS} ${LDFLAGS} -o ${PROJECT}
 
 debug:
-	cc ${PROJECT}.c ${CFLAGS} -g -o ${PROJECT}
+	${CC} ${PROJECT}.c ${CFLAGS} ${LDFLAGS} -g -o ${PROJECT}
 
 clean:
 	rm -rf ${PROJECT}
 
 install:
-	install ${PROJECT} ${PREFIX}/bin/${PROJECT}
+	install ${PROJECT} ${DESTDIR}${PREFIX}/bin/${PROJECT}
 
 uninstall:
 	rm -rf ${PREFIX}/bin/${PROJECT}