ctpv: fix build on Leopard and Snow Leopard

Closes: https://trac.macports.org/ticket/70170

graphics/ctpv/Portfile

@@ -3,7 +3,6 @@
 PortSystem          1.0
 PortGroup           github   1.0
 PortGroup           makefile 1.0
-PortGroup           compiler_blacklist_versions 1.0
 
 github.setup        NikitaIvanovV ctpv 1.1 v
 github.tarball_from archive
@@ -25,5 +24,6 @@ depends_lib-append  path:lib/libcrypto.dylib:openssl \
                     port:libmagic
 
 destroot.post_args  ${destroot.destdir} PREFIX=${prefix}
-compiler.blacklist-append \
-                    {clang < 1400} *gcc-4.*
+
+# https://trac.macports.org/ticket/70170
+compiler.cxx_standard 2020