qgis3: Use compiler_blacklist_versions portgroup

…instead of implementing the functionality manually.

See: https://trac.macports.org/ticket/55906

gis/qgis3/Portfile

@@ -2,6 +2,7 @@
 
 PortSystem          1.0
 PortGroup           cmake   1.1
+PortGroup           compiler_blacklist_versions 1.0
 PortGroup           github  1.0
 PortGroup           qt5     1.0
 
@@ -71,13 +72,10 @@ post-patch {
     # Handle QGIS Fix #11399 which doesn’t seem to work anymore
     reinplace -E "s|#ifdef Q_OS_MAC$|#if 0|" \
         ${worksrcpath}/src/app/layout/qgslayoutdesignerdialog.cpp
-
-    # Handle legacy OS/XCode
-    if {[vercmp $xcodeversion 4.6.3] <= 0} {
-        configure.compiler  llvm-gcc-4.2
-    }
 }
 
+compiler.blacklist      {clang < 500}
+
 cmake.install_prefix    ${applications_dir}
 # Plug-ins
 configure.args-append  "-DWITH_GRASS=OFF"