Create a gdk doctor check for MacPorts

Overview

Having MacPorts installed (especially old/outdated versions) can cause major issues when it comes to compiling software. A really common issue is trying to compile PostgreSQL:

Undefined symbols for architecture x86_64:
  "_ERR_put_error", referenced from:
      _be_tls_open_server in be-secure-openssl.o
  "_SSL_get_peer_certificate", referenced from:
      _be_tls_open_server in be-secure-openssl.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make[2]: *** [postgres] Error 1
make[1]: *** [all-backend-recurse] Error 2
make: *** [all-src-recurse] Error 2

Let's add a gdk doctor check to look for MacPorts and warn that it can cause issues.