[MacPorts] #61435: use_xcode default value is incorrect on macOS 11.0

[MacPorts]

#61435: use_xcode default value is incorrect on macOS 11.0
----------------------+----------------------------
  Reporter:  nicst35  |      Owner:  (none)
      Type:  defect   |     Status:  new
  Priority:  Normal   |  Milestone:  MacPorts 2.6.4
 Component:  base     |    Version:  2.6.99
Resolution:           |   Keywords:  bigsur
      Port:           |
----------------------+----------------------------

Comment (by ryandesign):

 https://developer.apple.com/documentation/macos-release-notes/macos-big-
 sur-11_0_1-release-notes

 > New in macOS Big Sur 11.0.1, the system ships with a built-in dynamic
 linker cache of all system-provided libraries. As part of this change,
 copies of dynamic libraries are no longer present on the filesystem. Code
 that attempts to check for dynamic library presence by looking for a file
 at a path or enumerating a directory will fail. Instead, check for library
 presence by attempting to dlopen() the path, which will correctly check
 for the library in the cache. (62986286)

-- 
Ticket URL: <https://trac.macports.org/ticket/61435#comment:5>
MacPorts <https://www.macports.org/>
Ports system for macOS