[MacPorts] #59679: Unison 2.51.2_2 + aqua doesn't build on MacOS 10.13

[MacPorts]

#59679: Unison 2.51.2_2 + aqua doesn't build on MacOS 10.13
------------------------------+-----------------------------
  Reporter:  brunoomartin     |      Owner:  (none)
      Type:  defect           |     Status:  new
  Priority:  Normal           |  Milestone:
 Component:  ports            |    Version:  2.6.2
Resolution:                   |   Keywords:  unison 2.51.2_2
      Port:  unison 2.51.2_2  |
------------------------------+-----------------------------

Comment (by brunoomartin):

 I do have Xcode installed with command line tools:
 # xcode-select -p
 /Applications/Xcode.app/Contents/Developer
 # gcc --version
 Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr
 --with-gxx-include-dir=/usr/include/c++/4.2.1
 Apple LLVM version 10.0.0 (clang-1000.11.45.5)
 Target: x86_64-apple-darwin17.7.0
 Thread model: posix
 InstalledDir:
 /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin

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Ticket URL: <https://trac.macports.org/ticket/59679#comment:2>
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