[MacPorts] #54773: port:libgcc/port:gcc7: proposed modifications, efficiency + libc++ support

[MacPorts]

#54773: port:libgcc/port:gcc7: proposed modifications, efficiency + libc++ support
-------------------------+-------------------------
 Reporter:  RJVB         |      Owner:
     Type:  enhancement  |     Status:  new
 Priority:  Normal       |  Milestone:
Component:  ports        |    Version:
 Keywords:  haspatch     |       Port:  gcc7 libgcc
-------------------------+-------------------------
 This ticket contains 2 patches that introduce a few modifications I've
 already submitted before in a ticket that has gone stale.
 FWIW, the changes look impressive in a unified diff, but they're really
 mostly indentation and a few extra if/else statements.

 Patchfile gcc7-libgcc.diff:

 - moves the C++ headerfiles into port:libgcc, and adds a symlink so they
 can be accessed via a version-independent path
 ($prefix/include/gcc/c++).\\This change means clang doesn't need to depend
 on a full gcc port to use -stdlib=macports-libstdc++
 - for those installing from source: adds a +with_libgcc variant to the
 main port and an automatic accompanying +stub variant to port:libgcc
 .\\The effect is simple: with these variants port:gcc7 will do a full
 install and libgcc becomes a simple stub so it can continue to be used as
 a 3rd party dependency. Users building from source will no longer need to
 build a good part of gcc twice, for the others nothing changes.
 - an unrelated minor change: make GCC invoke the clang assembler as
 discussed in #54771.

 Patchfile gcc7-libgcc+libcxx.diff:

 All of the above, plus a set of +libcxx variants which configure g++ to
 use libc++ and its associated headerfiles. These can be either taken from
 port:clang (3.9, 4.0 or 5.0) or from Xcode.
 Currently this is a hardwired modification; until now it has worked for
 everything I've tried to build with it (including a complex project like
 digiKam). Done properly (adding a -stdlib argument to the gcc driver) it
 will stand a good chance to be upstreamed but I'd like to give this a bit
 more real-world testing before I delve further into the gcc code base.
 (Target gcc8 for upstreaming?)

--
Ticket URL: <https://trac.macports.org/ticket/54773>
MacPorts <https://www.macports.org/>
Ports system for macOS