perl: add comments for patches (#469913)

let

  crossCompiling = !(stdenv.buildPlatform.canExecute stdenv.hostPlatform);

  commonPatches = [

    # Do not look in /usr etc. for dependencies.

    ./no-sys-dirs.patch

  ]

  # Fix build on Solaris on x86_64

  # See also:

  # * perl tracker: https://github.com/Perl/perl5/issues/9669

  # * netbsd tracker: https://gnats.netbsd.org/cgi-bin/query-pr-single.pl?number=44999

  ++ lib.optional stdenv.hostPlatform.isSunOS ./ld-shared.patch

  # Don't pass -no-cpp-precomp, even if it is "supported"

  #

  # cpp-precomp is a relic from NeXT days, when there was a separate

  # cpp preprocessor that produced "precompiled header files" (hence

  # cpp-precomp) - binary files containing information from .h files, that

  # sped up the compilation slightly.

  #

  # However, cpp-precomp was semi-broken and didn't accept a lot of valid

  # code, so there was a way to disable it (-no-cpp-precomp).

  #

  # This flag seems to have lost all relevance back in 2002 or so, when

  # cpp-precomp stopped being used; gcc dropped it completely in 4.8 (and

  # now errors out if you pass it), but for some reason clang still accepts

  # it (but it's a no-op). This means that if you compile perl with clang,

  # it will think that this flag is still supported, and it can cause issues

  # if you compile c code from perl:

  # https://trac.macports.org/ticket/38913

  #

  # More info about -no-cpp-precomp: https://www.mistys-internet.website/blog/blog/2013/10/19/no-cpp-precomp-the-compiler-flag-that-time-forgot

  #

  # See also: https://github.com/NixOS/nixpkgs/pull/1160

  ++ lib.optional stdenv.hostPlatform.isDarwin ./cpp-precomp.patch

  # Miniperl is a "bootstrap" perl which doesn't support dynamic loading, among other things.

  # It is used to build modules for the "final" perl.

  #

  # This patch enables more modules to be loaded with miniperl (mostly by

  # removing dynamically loaded libraries and using perl builtins instead),

  # which is needed during cross-compilation because the bootstrapping process

  # has more stages than the default build and doesn't get to the main perl

  # binary as early.

  #

  # Some more details: https://arsv.github.io/perl-cross/modules.html

  ++ lib.optional crossCompiling ./cross.patch;

  libc = if stdenv.cc.libc or null != null then stdenv.cc.libc else "/usr";