[MacPorts] #54198: upgrade: port:gcc7

[MacPorts]

#54198: upgrade: port:gcc7
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 Reporter:  RJVB      |      Owner:
     Type:  update    |     Status:  new
 Priority:  Normal    |  Milestone:
Component:  ports     |    Version:
 Keywords:  haspatch  |       Port:  gcc7
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 This ticket provides patches to upgrade port:gcc7 from a recent beta
 version to the 7.1.0 release version. Tested on OS X 10.9.5 .

 Gcc 7.1.0 comes with a new libgfortran; this ticket does not provide the
 patch(es) required to adapt port:gcc6 (and earlier version?) to add a
 port:libgcc6 (I'm curious why the libgcc ports handle ABI changes this way
 while all other ports just expect dependents to revbump and rebuild?)

 I've added 1 small patch which makes gcc c.s. invoke the cctools assembler
 with the `-q` option. This option makes `as` invoke the clang integrated
 assembler instead of running its own obsolete GNU-as based routines; the
 option is supposedly the default but that is clearly not the case in
 practice. It does indicate that the option must be safe to use; the effect
 is that the GCC compilers will use an up-to-date assembler with support
 for instruction sets unknown to Apple's GNU as fork.

 I'm including a 2nd Portfile patch, to be applied to the upgraded Portfile
 which I think should be incorporated at the same time as the upgrade. This
 provides a PoC implementation of the variant I suggested to roll back the
 standard runtime libraries into the main port, and turn port:libgcc into a
 stub that exposes the runtime dylibs into $prefix/lib/libgcc. This cuts
 install and upgrade times roughly in half for users who install from
 source, which is all the more interesting to those who need the universal
 variant. It requires no functionality that should be part of "Base", it
 just inverts the symlinking logic.

 Concretely, interested users can install port:gcc7 with the `+with_libgcc`
 variant (name open for improvement). This simply removes the port's
 dependency on port:libgcc and shunts the post-destroot code replacing the
 dylibs concerned with symlinks.
 Then, when port:libgcc is required or upgraded, the `with_libgcc` variant
 is detected. That leads to the auto-activation of a `+stub` variant, which
 adds a dependency on port:gcc7, drops all source-related phases and re-
 define the destroot phase to create symlinks in $prefix/lib/libgcc that
 point to the dylibs in $prefix/lib/gcc7.
 (I considered using the same variant for both ports but failed to find a
 term that makes sense for both).

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Ticket URL: <https://trac.macports.org/ticket/54198>
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