Allow CFLAGS additions and install in DESTDIR

This make it easier to create a Debian package.

Makefile

@@ -5,7 +5,7 @@ STRIP=strip
 
 OUTDIR?=build
 OUTFILE="$(OUTDIR)/rtl_wmbus"
-CFLAGS?=-Iinclude -std=gnu99
+CFLAGS+=-Iinclude -std=gnu99
 CFLAGS_WARNINGS?=-Wall -W -Waggregate-return -Wbad-function-cast -Wcast-align -Wcast-qual -Wchar-subscripts -Wcomment -Wno-float-equal -Winline -Wmain -Wmissing-noreturn -Wno-missing-prototypes -Wparentheses -Wpointer-arith -Wredundant-decls -Wreturn-type -Wshadow -Wsign-compare -Wstrict-prototypes -Wswitch -Wunreachable-code -Wno-unused -Wuninitialized
 LIB?=-lm
 SRC=rtl_wmbus.c
@@ -62,7 +62,8 @@ pi1:
 rebuild: clean all
 
 install: release
-	cp -f $(OUTFILE) /usr/bin
+	install -d $(DESTDIR)/usr/bin
+	install $(OUTFILE) $(DESTDIR)/usr/bin
 
 clean:
 	$(RM) -rf "$(OUTDIR)"