bin/package: reallow building old ksh versions (re: 6cc2f6a0)

When building old code for debugging purposes (e.g. when doing 'git
bisect' runs), it's best to use the current build system even with
the old code, because the old build system was very broken. E.g.:

	git checkout (some old commit)
	git checkout master bin src/cmd/INIT # use new build system
	bin/package make

However, that became impossible in 6cc2f6a0 because the new
SHOPT.sh script was unconditionally sourced. The error caused the
script to exit because '.' is a special builtin.

bin/package, src/cmd/INIT/package.sh:
- If src/cmd/ksh93/SHOPT.sh doesn't exist, issue a warning instasd
  of trying to source it.

README.md

@@ -47,7 +47,7 @@ Though there are many bugs left to fix, we are confident at this point that
 ## Build
 
 To build ksh with a custom configuration of features, edit
-[`src/cmd/ksh93/SHOPT.sh`](/ksh93/ksh/blob/master/src/cmd/ksh93/SHOPT.sh).
+[`src/cmd/ksh93/SHOPT.sh`](https://github.com/ksh93/ksh/blob/master/src/cmd/ksh93/SHOPT.sh).
 
 Then `cd` to the top directory and run:
 ```sh

bin/package

@@ -1436,7 +1436,11 @@ SHOPT()
 	esac
 }
 ksh_shoptflags=
-. src/cmd/ksh93/SHOPT.sh	# this script calls SHOPT() to set options
+shopt_sh='src/cmd/ksh93/SHOPT.sh'	# this script calls SHOPT() to set options
+if	test -f "$shopt_sh"
+then	. "$shopt_sh"
+else	echo "WARNING: $shopt_sh is missing" >&2
+fi
 case $ksh_shoptflags in
 ?*)	# add the extra flags as an argument to mamake
 	assign="${assign:+$assign }KSH_SHOPTFLAGS=\"\$ksh_shoptflags\"" ;;

src/cmd/INIT/package.sh

@@ -1435,7 +1435,11 @@ SHOPT()
 	esac
 }
 ksh_shoptflags=
-. src/cmd/ksh93/SHOPT.sh	# this script calls SHOPT() to set options
+shopt_sh='src/cmd/ksh93/SHOPT.sh'	# this script calls SHOPT() to set options
+if	test -f "$shopt_sh"
+then	. "$shopt_sh"
+else	echo "WARNING: $shopt_sh is missing" >&2
+fi
 case $ksh_shoptflags in
 ?*)	# add the extra flags as an argument to mamake
 	assign="${assign:+$assign }KSH_SHOPTFLAGS=\"\$ksh_shoptflags\"" ;;