diff --git a/Makefile b/Makefile
index dd77985..3a9c844 100644
--- a/Makefile
+++ b/Makefile
@@ -13,18 +13,18 @@
 .PRAGMA: posix_202x # future POSIX standard support à la pdpmake(1)
 .PRAGMA: command_comment # breaks without this?
 
-DESTDIR=./dist
-PREFIX=/usr/local
+DESTDIR ?= dist
+PREFIX ?= /usr/local
 
-SYSEXITS!=printf '\043include <sysexits.h>\n' | cpp -M - | sed 's/ /\n/g' \
-	| sed -n 's/sysexits\.h//p' || printf 'include/\n'
+SYSEXITS != printf '\043include <sysexits.h>\n' | cpp -M - | sed 's/ /\n/g' \
+	| sed -n 's/sysexits\.h//p' || printf 'include\n'
 
-CC?=cc
-RUSTC?=rustc
-RUSTLIBS=--extern getopt=build/o/libgetopt.rlib \
+CC ?= cc
+RUSTC ?= rustc
+RUSTLIBS = --extern getopt=build/o/libgetopt.rlib \
 	--extern sysexits=build/o/libsysexits.rlib \
 	--extern strerror=build/o/libstrerror.rlib
-CFLAGS+=-I$(SYSEXITS)
+CFLAGS += -I$(SYSEXITS)
 
 .PHONY: all
 all: dj false fop hru intcmp rpn scrut str strcmp true
@@ -36,12 +36,12 @@ build:
 
 .PHONY: clean
 clean:
-	rm -rf build/ dist/
+	rm -rf build dist
 
 dist: all
-	mkdir -p $(DESTDIR)$(PREFIX)/bin $(DESTDIR)$(PREFIX)/share/man/man1
-	cp build/bin/* $(DESTDIR)$(PREFIX)/bin/
-	cp docs/*.1 $(DESTDIR)$(PREFIX)/share/man/man1/
+	mkdir -p $(DESTDIR)/$(PREFIX)/bin $(DESTDIR)/$(PREFIX)/share/man/man1
+	cp build/bin/* $(DESTDIR)/$(PREFIX)/bin
+	cp docs/*.1 $(DESTDIR)/$(PREFIX)/share/man/man1
 
 .PHONY: install
 install: dist