move install to Makefile; add install for man

Justfile

@@ -5,10 +5,5 @@ exec *ARGS: build
 	build/args {{ARGS}}
 build:
 	@make build
-
-PREFIX := if shell('id -u') == "0" { '/usr/local' } else { x'$HOME/.local' }
-
 install:
-	install -CDm0755 -t {{ PREFIX }}/bin/ build/{{ NAME }}
-	# install -D -t {{ PREFIX }}/share/man/1 build/{{ NAME }}
-
+	@make install

Makefile

@@ -21,6 +21,17 @@ build: build/ build/$(NAME)
 build/$(NAME): src/main.c $(HEADERS)
 	cc -o $@ $<
 
+define PREFIX
+ifeq $(UID) 0
+	/usr/local
+else
+	$(HOME)/.local
+endif
+endef
+
+install:
+	install -CDm0755 -t $(PREFIX)/bin/ build/$(NAME)
+	install -CDm0644 -t  $(PREFIX)/share/man/man1 build/$(NAME)
 
 test:
 	$(error testing using `make' not supported; use `just')