feat: improve makefile

- remove compile rule (implicit)
- no dependency tracking for install/check

Makefile

@@ -1,17 +1,20 @@
-CC=gcc
+CC?=gcc
 CFLAGS+=-D_GNU_SOURCE
-LIBS+=-lskarnet
-BINDIR=/usr/bin
-MANDIR=/usr/share/man
+LDLIBS+=-lskarnet
+PREFIX?=/usr
+BINDIR=$(PREFIX)/bin
+MANDIR=$(PREFIX)/share/man
 
 all: fdcreate
 
-fdcreate: fdcreate.o
-	$(CC) -o fdcreate fdcreate.o $(LIBS)
+clean:
+	@rm -f fdcreate fdcreate.o
 
-check: fdcreate
+check:
 	./fdcreate test [ -f /proc/self/fd/3 ]
 
-install: fdcreate
+install:
 	install -Dm755 fdcreate -t $(DESTDIR)$(BINDIR)
 	install -Dm644 fdcreate.1 -t $(DESTDIR)$(MANDIR)/man1
+
+.PHONY := install check clean all