pythia8: Include patch for C++20 / Clang

[paulgessinger]

Pythia8 vendors some FJCore sources that are as of Pythia8 312 incompatible with C++20 on clang. This adds a patch that makes it compatible in these scenarios.
Related issue on the Pythia repository: https://gitlab.com/Pythia8/releases/-/issues/523

This PR:

• Moves the existing modification of configure to generate a modified Makefile from a patch function to the already existing function being run before the configure step
  • This is needed because defining a patch function prevents using patch() in the class (as far as I can tell)
• Adds patches for %clang and %appleclang when cxxstd=20 and `@:8312

---

Compilation on linux:

$ spack install pythia8@8.312%clang cxxstd=20
==> Installing pythia8-8.312-zxo6h4jqqw4yjgzzythtyehp2yyzjvws [14/14]
==> No binary for pythia8-8.312-zxo6h4jqqw4yjgzzythtyehp2yyzjvws found: installing from source
==> Using cached archive: /scratch/pagessin/spack-test/spack/var/spack/cache/_source-cache/archive/ba/bad98e2967b687046c4568c9091d630a0c31b628745c021a994aba4d1d50f8ea.tgz
==> Applied patch /scratch/pagessin/spack-test/spack/var/spack/repos/builtin/packages/pythia8/pythia8-cpp20-fjcore-forward-decl.patch
==> pythia8: Executing phase: 'autoreconf'
==> pythia8: Executing phase: 'configure'
==> pythia8: Executing phase: 'build'
==> pythia8: Executing phase: 'install'
==> pythia8: Successfully installed pythia8-8.312-zxo6h4jqqw4yjgzzythtyehp2yyzjvws
  Stage: 0.48s.  Autoreconf: 0.00s.  Configure: 1.52s.  Build: 30.28s.  Install: 0.60s.  Post-install: 0.29s.  Total: 33.33s
[+] /scratch/pagessin/spack-test/spack/opt/spack/linux-almalinux9-zen2/clang-17.0.6/pythia8-8.312-zxo6h4jqqw4yjgzzythtyehp2yyzjvws

Compilation on macOS:

$ spack install pythia8@8.312 cxxstd=20
==> Installing pythia8-8.312-otexj543luj3kaglxp2hlxx3ojiz2wfh [13/13]
==> No binary for pythia8-8.312-otexj543luj3kaglxp2hlxx3ojiz2wfh found: installing from source
==> Using cached archive: /Users/pagessin/spack/var/spack/cache/_source-cache/archive/ba/bad98e2967b687046c4568c9091d630a0c31b628745c021a994aba4d1d50f8ea.tgz
==> Applied patch /Users/pagessin/spack/var/spack/repos/builtin/packages/pythia8/pythia8-cpp20-fjcore-forward-decl.patch
==> pythia8: Executing phase: 'autoreconf'
==> pythia8: Executing phase: 'configure'
==> pythia8: Executing phase: 'build'
==> pythia8: Executing phase: 'install'
==> pythia8: Successfully installed pythia8-8.312-otexj543luj3kaglxp2hlxx3ojiz2wfh
  Stage: 0.40s.  Autoreconf: 0.00s.  Configure: 2.90s.  Build: 48.19s.  Install: 1.22s.  Post-install: 0.19s.  Total: 53.05s
[+] /Users/pagessin/spack/opt/spack/darwin-sequoia-m1/apple-clang-15.0.0/pythia8-8.312-otexj543luj3kaglxp2hlxx3ojiz2wfh

[paulgessinger]

fyi @vvolkl

[vvolkl]

Very nice, thanks!