gnu: doxygen: Fix build for the Hurd.

* gnu/packages/patches/doxygen-hurd.patch: New file.
* gnu/local.mk (dist_patch_DATA): Add it.
* gnu/packages/documentation.scm (doxygen)[arguments]: When building for the
Hurd, add new phase 'apply-patch' and use it.

gnu/packages/patches/doxygen-hurd.patch

@@ -0,0 +1,30 @@
+Upstream status: Adapted from upstream.
+
+From d3d968e5835f449d7ea715f45160db81ea906303 Mon Sep 17 00:00:00 2001
+From: Samuel Thibault <samuel.thibault@ens-lyon.org>
+Date: Tue, 16 Aug 2022 20:29:54 +0200
+Subject: [PATCH] Fix build on GNU/Hurd
+
+There is no path length limitation there, even via pathconf.  But glibc
+provides a getcwd function that allocates the buffer dynamically so we can
+just leverage that.
+---
+ include/ghc/filesystem.hpp | 7 +++++++
+ 1 file changed, 7 insertions(+)
+
+--- a/filesystem/filesystem.hpp.orig	2022-08-11 22:14:32.000000000 +0200
++++ b/filesystem/filesystem.hpp	2023-06-13 07:26:25.310000000 +0200
+@@ -4081,6 +4081,13 @@
+         return path();
+     }
+     return path(std::wstring(buffer.get()), path::native_format);
++#elif defined(__GLIBC__)
++    std::unique_ptr<char, decltype(&std::free)> buffer { ::getcwd(NULL, 0), std::free };
++    if (buffer == nullptr) {
++        ec = detail::make_system_error();
++        return path();
++    }
++    return path(buffer.get());
+ #else
+     size_t pathlen = static_cast<size_t>(std::max(int(::pathconf(".", _PC_PATH_MAX)), int(PATH_MAX)));
+     std::unique_ptr<char[]> buffer(new char[pathlen + 1]);