Use LDFLAGS to ensure we link with MacPorts libcurl not macOS libcurl.
https://github.com/egallesio/STklos/issues/505
--- extensions/curl/lib/stklos/Makefile.in.orig	2023-03-07 05:24:11.000000000 -0600
+++ extensions/curl/lib/stklos/Makefile.in	2023-03-16 23:39:19.000000000 -0500
@@ -570,7 +570,7 @@
 	@CC@ @CFLAGS@  @STKCFLAGS@ @SH_COMP_FLAGS@ \
          -I$(BASEDIR)/src @GCINC@ @GMPINC@ \
          -c -o $*.o $*.c
-	@SH_LOADER@ @SH_LOAD_FLAGS@ -o $*.@SH_SUFFIX@ $*.o @DLLIBS@ -lcurl
+	@SH_LOADER@ @SH_LOAD_FLAGS@ -o $*.@SH_SUFFIX@ $*.o @DLLIBS@ $(LDFLAGS) -lcurl
 	/bin/rm -f $*.o
 
 $(EXTENSION)-incl.c: $(EXTENSION).stk $(SCM_SRCS)