Don't use -rpath. It's not needed in MacPorts and causes an error with
older compilers.
--- autosetup/proj.tcl	2025-05-29 22:35:12.000000000 +0800
+++ autosetup/proj.tcl	2025-05-31 14:51:36.000000000 +0800
@@ -1117,6 +1117,8 @@
 # from exec-prefix by default).
 #
 proc proj-check-rpath {} {
+  define LDFLAGS_RPATH ""
+  return 0
   if {[proj-opt-was-provided libdir]
       || [proj-opt-was-provided exec-prefix]} {
     set lp "[get-define libdir]"