diff -u a/Makefile b/Makefile
--- a/Makefile	2020-12-27 18:48:53.934098765 +0100
+++ b/Makefile	2020-12-27 18:50:44.022674117 +0100
@@ -19,13 +19,13 @@
 # http://www.gnu.org/prep/standards/html_node/Directory-Variables.html
 PREFIX ?= /usr/local
 EXEC_PREFIX = $(PREFIX)
-BINDIR = $(EXEC_PREFIX)/bin
+BINDIR ?= $(EXEC_PREFIX)/bin
 DATAROOTDIR = $(PREFIX)/share
 DOCDIR = $(DATAROOTDIR)/doc/$(CPROG)
 SYSCONFDIR ?= $(PREFIX)/etc
 HTMLDIR = $(DOCDIR)
-INCLUDEDIR = $(DESTDIR)$(PREFIX)/include
-LIBDIR = $(DESTDIR)$(EXEC_PREFIX)/lib
+INCLUDEDIR ?= $(DESTDIR)$(PREFIX)/include
+LIBDIR ?= $(DESTDIR)$(EXEC_PREFIX)/lib
 PID_FILE ?= /var/run/$(CPROG).pid
 
 # build tools
@@ -337,10 +337,10 @@
 	install -m 755 $(CPROG) "$(BINDIR)/"
 
 install-headers:
-	install -m 644 $(HEADERS) "$(INCLUDEDIR)"
+	install -m 644 $(HEADERS) "$(INCLUDEDIR)/"
 
 install-lib: lib$(CPROG).a
-	install -m 644 $< "$(LIBDIR)"
+	install -m 644 $< "$(LIBDIR)/"
 
 install-slib: lib$(CPROG).so
 	$(eval version=$(shell grep -w "define CIVETWEB_VERSION" include/civetweb.h | sed 's|.*VERSION "\(.*\)"|\1|g'))