[MacPorts] #58260: gcc{6, 7, 8, 9} : Build failures due to "Atomic" error with macOS 10.4.4 and Xcode 10.2

[MacPorts]

#58260: gcc{6,7,8,9} : Build failures due to "Atomic" error with macOS 10.4.4 and
Xcode 10.2
-------------------------------------+-------------------------------------
  Reporter:  physicsbeany            |      Owner:  Nullstelllensatz
                                     |  <50169837+Nullstelllensatz@…>
      Type:  defect                  |     Status:  closed
  Priority:  Normal                  |  Milestone:
 Component:  ports                   |    Version:  2.5.4
Resolution:  fixed                   |   Keywords:  mojave
      Port:  gcc9 gcc8 gcc7 gcc6     |
  libgcc-devel libgcc8 libgcc7       |
  libgcc6                            |
-------------------------------------+-------------------------------------

Comment (by jmon12):

 Dear all,
 I need to build gcc5 from source and fail encountering the so-called
 "Atomic error". From what I understand, it seems that the "xcode-bug-
 _Atomic-fix.patch" is applied based on the value of the `xcodeversion`
 variable. See lines 84:90 in gcc5's portfile:
 {{{
 if {[vercmp ${xcodeversion} 10.2] >= 0} {
     # https://trac.macports.org/ticket/58260
     # Patch for Xcode bug, taken from
     # https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89864#c43
     # https://github.com/Homebrew/homebrew-core/pull/39134/files
     patchfiles-append   xcode-bug-_Atomic-fix.patch
 }
 }}}

 I don't know how the `xcodeversion` variable is defined, but it's probably
 not defined if Xcode is not installed, which is my case. I'm indeed using
 only the command line tools.

 So:
 * It doesn't concern full xcode installations only
 * Which condition should be evaluated instead? I'll try to investigate
 myself but am at the very beginning of discovering macports' internals...

 From a superficial google search, I'm able to get the version of the CLT
 using `pkgutil --pkg-info=com.apple.pkg.CLTools_Executables` giving in my
 case:
 {{{
 package-id: com.apple.pkg.CLTools_Executables
 version: 10.3.0.0.1.1562985497
 volume: /
 location: /
 install-time: 1563885516
 groups: com.apple.FindSystemFiles.pkg-group
 }}}

 Is there another macports variable based on the above CLT version number
 instead of the xcode's one (using `xcodebuild`)?

 I would be happy to contribute, I'm sorry for my lack of knowledge. Hope
 I'll be at some point able to give back to the macports community!

-- 
Ticket URL: <https://trac.macports.org/ticket/58260#comment:27>
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