bpo-43568: Drop support for MACOSX_DEPLOYMENT_TARGET < 10.3 (GH-25827)

Only complain if the config target is >= 10.3 and the current target is
< 10.3. The check was originally added to ensure that incompatible
LDSHARED flags are not used, because -undefined dynamic_lookup is
used when building for 10.3 and later, and is not supported on older OS
versions. Apart from that, there should be no problem in general
with using an older target.

Authored-by: Joshua Root <jmr@macports.org>
diff --git a/configure b/configure
index 86591e2..e3e6dad 100755
--- a/configure
+++ b/configure
@@ -9656,19 +9656,7 @@
 		   test ${dep_target_minor} -le 2
 		then
 			# building for OS X 10.0 through 10.2
-			LDSHARED='$(CC) -bundle'
-			LDCXXSHARED='$(CXX) -bundle'
-			if test "$enable_framework" ; then
-				# Link against the framework. All externals should be defined.
-				BLDSHARED="$LDSHARED "'$(PYTHONFRAMEWORKDIR)/Versions/$(VERSION)/$(PYTHONFRAMEWORK)'
-				LDSHARED="$LDSHARED "'$(PYTHONFRAMEWORKPREFIX)/$(PYTHONFRAMEWORKDIR)/Versions/$(VERSION)/$(PYTHONFRAMEWORK)'
-				LDCXXSHARED="$LDCXXSHARED "'$(PYTHONFRAMEWORKPREFIX)/$(PYTHONFRAMEWORKDIR)/Versions/$(VERSION)/$(PYTHONFRAMEWORK)'
-			else
-				# No framework, use the Python app as bundle-loader
-				BLDSHARED="$LDSHARED "'-bundle_loader $(BUILDPYTHON)'
-				LDSHARED="$LDSHARED "'-bundle_loader $(BINDIR)/python$(VERSION)$(EXE)'
-				LDCXXSHARED="$LDCXXSHARED "'-bundle_loader $(BINDIR)/python$(VERSION)$(EXE)'
-			fi
+			as_fn_error $? "MACOSX_DEPLOYMENT_TARGET too old ($MACOSX_DEPLOYMENT_TARGET), only 10.3 or later is supported" "$LINENO" 5
 		else
 			# building for OS X 10.3 and later
 			LDSHARED='$(CC) -bundle -undefined dynamic_lookup'