No such thing as MACOSX_DEVELOPMENT_TARGET.

This appears to be a typo that has been with us since 69625aa
and the real name is MACOSX_DEPLOYMENT_TARGET.

So do the same thing the MacPorts folks have been doing, meaning
this is just the "fix-ld-modification.patch" from:

https://trac.macports.org/browser/trunk/dports/lang/perl5/files/5.24?rev=148407

hints/darwin.sh

@@ -294,7 +294,7 @@ case "$osvers" in  # Note: osvers is the kernel version, not the 10.x
 [7-9].*)   # OS X 10.3.x - 10.5.x
    lddlflags="${ldflags} -bundle -undefined dynamic_lookup"
    case "$ld" in
-       *MACOSX_DEVELOPMENT_TARGET*) ;;
+       *MACOSX_DEPLOYMENT_TARGET*) ;;
        *) ld="env MACOSX_DEPLOYMENT_TARGET=10.3 ${ld}" ;;
    esac
    ;;