classias: Error early with libc++

Update error message to indicate that libc++, not Mavericks, is the reason the
port can't build.

math/classias/Portfile

@@ -24,12 +24,12 @@ depends_lib         port:liblbfgs
 compiler.blacklist  *clang*
 
 platform darwin {
-    if {${os.major} >= 13} {
+    if {${configure.cxx_stdlib} eq "libc++"} {
         depends_lib
         depends_run
         pre-fetch {
-            ui_error "$name does not build on Mavericks or later."
-            error "unsupported platform"
+            ui_error "$name does not build with libc++"
+            return -code error "unsupported platform"
         }
     }
 }