Don't use -rpath. It's not needed in MacPorts and causes an error with
older compilers.
--- autosetup/proj.tcl.orig	2025-02-06 07:59:25.000000000 -0600
+++ autosetup/proj.tcl	2025-02-08 18:16:09.000000000 -0600
@@ -918,6 +918,8 @@
 # checked here will work but then fails at build-time, and the current
 # order of checks reflects that.
 proc proj-check-rpath {} {
+  define LDFLAGS_RPATH ""
+  return 0
   set rc 1
   if {[proj-opt-was-provided libdir]
       || [proj-opt-was-provided exec-prefix]} {