qgis: Use compiler_blacklist_versions portgroup

…instead of implementing the functionality manually.

See: https://trac.macports.org/ticket/55905

gis/qgis/Portfile

@@ -2,6 +2,7 @@
 
 PortSystem          1.0
 PortGroup           cmake   1.0
+PortGroup           compiler_blacklist_versions 1.0
 #PortGroup           github  1.0
 PortGroup           qt4     1.0
 
@@ -60,12 +61,10 @@ post-patch {
     reinplace -E "s|@@@|${prefix}|g" \
         ${worksrcpath}/mac/app.info.plist.in
 #    reinplace -E "s|Clang|AppleClang|" ${worksrcpath}/CMakeLists.txt
-    # Handle legacy OS/XCode
-    if {[vercmp $xcodeversion 4.6.3] <= 0} {
-        configure.compiler  llvm-gcc-4.2
-    }
 }
 
+compiler.blacklist  {clang < 500}
+
 set Py_FRM              ${frameworks_dir}/Python.framework/Versions/2.7
 
 #configure.args-append  "--trace"