l2md, Makefile: add PREFIX flag

This allows the user to choose a different installation directory.

Signed-off-by: Xinglu Chen <public@yoctocell.xyz>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
diff --git a/Makefile b/Makefile
index a189938..a7c9002 100644
--- a/Makefile
+++ b/Makefile
@@ -1,14 +1,15 @@
-CFLAGS = -O2 -Wall -Werror
+CFLAGS  = -O2 -Wall -Werror
 LDFLAGS = -lgit2
+PREFIX ?= /usr/bin
 
 l2md: l2md.o config.o env.o utils.o repo.o mail.o maildir.o pipe.o
 	$(CC) -o $@ $^ $(LDFLAGS)
 
 install:
-	cp l2md /usr/bin/l2md
+	cp l2md $(PREFIX)/l2md
 
 uninstall:
-	$(RM) /usr/bin/l2md
+	$(RM) $(PREFIX)/l2md
 
 clean:
 	$(RM) *.o l2md