glib: fix docs cross compilation (again)

Do not depend on host gtk-doc when cross compiling (we do not use it
there), and document why we need a host gtk-doc in the first place.

pkgs/development/libraries/glib/default.nix

···
   buildInputs = [
     libelf setupHook pcre
     bash gnum4 # install glib-gettextize and m4 macros for other apps to use
-    gtk-doc
   ] ++ optionals stdenv.isLinux [
     libselinux
     util-linuxMinimal # for libmount
   ] ++ optionals stdenv.isDarwin (with darwin.apple_sdk.frameworks; [
     AppKit Carbon Cocoa CoreFoundation CoreServices Foundation
-  ]);
+  ]) ++ optionals (stdenv.hostPlatform == stdenv.buildPlatform) [
+    # Note: this needs to be both in buildInputs and nativeBuildInputs. The
+    # Meson gtkdoc module uses find_program to look it up (-> build dep), but
+    # glib's own Meson configuration uses the host pkg-config to find its
+    # version (-> host dep). We could technically go and fix this in glib, add
+    # pkg-config to depsBuildBuild, but this would be a futile exercise since
+    # Meson's gtkdoc integration does not support cross compilation[1] anyway
+    # and this derivation disables the docs build when cross compiling.
+    #
+    # [1] https://github.com/mesonbuild/meson/issues/2003
+    gtk-doc
+  ];
 
   strictDeps = true;