Honor MacPorts CXXFLAGS.
--- ./setup.py	2023-12-14 00:49:03
+++ ./setup.py	2023-12-14 00:57:38
@@ -79,6 +79,9 @@
         c_sources.append(source)
     extra_args = ext.extra_compile_args or []
 
+    cxxflags = os.environ['CXXFLAGS'].split()
+    extra_args.extend(cxxflags)
+
     objects = []
 
     macros = ext.define_macros[:]