Fix portability problems going to OpenBSD

The earlier Makefile assumed GNU tooling in the following ways:

1. install having a -t target directory argument.
2. make having an internal rule to make a target based only on...
   ansi2text: ansi2text.o
3. man pages go to /usr/local/share/man, which is not the location
   on OpenBSD

Made a note about installation, pointing out the use of the MANDIR
variable to correct the man page destination.

Added an uninstall target.

Makefile

@@ -12,11 +12,16 @@ MANDIR=$(PREFIX)/share/man
 all: ansi2text
 
 ansi2text: ansi2text.o
+	$(CC) -o $@ $>
 
 install: ansi2text
 	install -d $(BINDIR) $(MANDIR) $(MANDIR)/man1
-	install -m 755 -t $(BINDIR) ansi2text
-	install -m 444 -t $(MANDIR)/man1 ansi2text.1
+	install -m 755 ansi2text $(BINDIR)
+	install -m 444 ansi2text.1 $(MANDIR)/man1
 
 clean:
 	-rm ansi2text ansi2text.o
+
+uninstall:
+	-rm $(BINDIR)/ansi2text
+	-rm $(MANDIR)/man1/ansi2text.1

README

@@ -5,3 +5,14 @@ Nathanial Sloss where he included its source. See
 netbsd_gnat_change_request.origin which was saved from
 https://gnats.netbsd.org/48092
 
+Installation:
+
+make
+sudo make install
+
+Note: on some systems you may want to set MANDIR, e.g. ...
+
+sudo make install MANDIR=/usr/local/man
+
+The default is to put the man page under /usr/local/share/man.
+See Makefile.