Support --prefix and --mandir in configure script

README.7

@@ -93,12 +93,12 @@ macOS
 and Linux.
 On
 .Ox ,
-additionally set
-.Ev MANDIR=/usr/local/man .
+configure with
+.Fl Fl mandir=/usr/local/man .
 .Bd -literal -offset indent
 \&./configure
 make all
-sudo make install PREFIX=/usr/local
+sudo make install
 .Ed
 .
 .Pp

configure

@@ -22,6 +22,14 @@ defvar() {
 
 exec >config.mk
 
+for opt; do
+	case "${opt}" in
+		(--prefix=*) echo "PREFIX = ${opt#*=}" ;;
+		(--mandir=*) echo "MANDIR = ${opt#*=}" ;;
+		(*) echo "warning: unsupported option ${opt}" >&2 ;;
+	esac
+done
+
 case "$(uname)" in
 	(FreeBSD)
 		ldlibs -lncursesw