[MacPorts] #54818: clang-5.0 depends on gcc6 instead of gcc7

[MacPorts]

#54818: clang-5.0 depends on gcc6 instead of gcc7
------------------------+-----------------------
 Reporter:  mouse07410  |      Owner:
     Type:  update      |     Status:  new
 Priority:  Normal      |  Milestone:
Component:  ports       |    Version:
 Keywords:              |       Port:  clang-5.0
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 Both clang-5.0 and gcc7 support C++17, which is the main reason I need
 these ports now.

 However clang-5.0 shows gcc6 as its library dependency - thus forcing me
 to spend almost 1GB on the already pretty much filled-up disk.

 Coud you please update clang-5.0 dependencies in such a way that it would
 accept both gcc6 and gcc7 (i.e., won't install gcc6 is gcc7 is already
 installed), or just require gcc7?

 Thanks!


 {{{
 $ port info clang-5.0
 clang-5.0 @5.0.0_2 (lang)
 Variants:             [+]analyzer, assertions, debug, [+]libstdcxx,
 universal

 Description:          Clang is an LLVM native C/C++/Objective-C compiler,
 which aims to deliver amazingly fast compiles
                       (e.g. about 3x faster than GCC when compiling
 Objective-C code in a debug configuration),
                       extremely useful error and warning messages and to
 provide a platform for building great source
                       level tools. The included Clang Static Analyzer is a
 tool that automatically finds bugs in your
                       code, and is a great example of the sort of tool
 that can be built using the Clang frontend as a
                       library to parse C/C++ code.
 Homepage:             http://clang.llvm.org/

 Extract Dependencies: xz
 Build Dependencies:   cmake
 Library Dependencies: libxml2, libomp, llvm-5.0, python27, libedit,
 libffi, ncurses, zlib, libcxx, gcc6
 Runtime Dependencies: clang_select, ld64, perl5
 Platforms:            darwin
 License:              NCSA
 Maintainers:          Email: jeremyhu at macports.org, GitHub: jeremyhu
                       Email: larryv at macports.org
 $
 }}}

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Ticket URL: <https://trac.macports.org/ticket/54818>
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