re-enable notmuch.sym generation using POSIX sed

Unfortunately Robin Green's patch 52e4dedf9a was lost when I created
gen-version-script.sh. This merges his changes manually into that
script. It turns out tabs seem not needed in version script
files, so I simplified a bit and removed the printf.

Thanks to Alexander Botero-Lowry for help and testing.

lib/gen-version-script.sh

@@ -23,5 +23,5 @@ while read sym; do
 	    ;;
     esac
 done
-sed  -n 's/^\s*\(notmuch_[a-z_]*\)\s*(.*/\t\1;/p' $HEADER
+sed  -n 's/^[[:space:]]*\(notmuch_[a-z_]*\)[[:space:]]*(.*/ \1;/p' $HEADER
 printf "local: *;\n};\n"