RFE: gcc-10+: enable zstd support

[cooljeanius (Eric Gallager)]

As of GCC 10, GCC now supports zstd compression of LTO bytecode: ​https://gcc.gnu.org/gcc-10/changes.html

This can be enabled with the --with-zstd option: ​https://gcc.gnu.org/install/prerequisites.html

Trace mode also shows the configure process trying to access zstd:

Warning: The following existing files were hidden from the build system by trace mode:
  /opt/local/Library/Frameworks/Python.framework/Versions/3.7/bin/sphinx-build
  /opt/local/bin/aclocal
  /opt/local/bin/autoconf
  /opt/local/bin/autoheader
  /opt/local/bin/bison
  /opt/local/bin/clang-mp-10
  /opt/local/bin/ggrep
  /opt/local/bin/gmkdir
  /opt/local/include/mach-o/loader.h
  /opt/local/include/valgrind/memcheck.h
  /opt/local/include/zstd.h
  /opt/local/lib/libunwind.dylib
  /opt/local/lib/libzstd.a
  /opt/local/lib/libzstd.dylib

All that needs to be done to allow this is to add a port:zstd entry to the depends_lib-append section and a line for --with-zstd=${prefix} to the configure.args section.

[MarcusCalhoun-Lopez (Marcus Calhoun-Lopez)]

There is a ​pull request that attempts to fix this problem.

[MarcusCalhoun-Lopez (Marcus Calhoun-Lopez)]

In 6d57b4f979f2dfede4c2b0b383967c0f02102507/macports-ports (master):

gcc10+: recognize opportunistic use of zstd port

Fixes #63750