meson: Patch out default boost search paths. See #86131.

Avoids impure builds on unsandboxed non-NixOS builds, see:
https://github.com/NixOS/nixpkgs/issues/86131#issuecomment-711051774

pkgs/development/tools/build-managers/meson/boost-Do-not-add-system-paths-on-nix.patch

@@ -0,0 +1,40 @@
+From 536108b10271f2f42d41c7d9ddb4ce2ea1851f4f Mon Sep 17 00:00:00 2001
+From: =?UTF-8?q?Niklas=20Hamb=C3=BCchen?= <mail@nh2.me>
+Date: Sat, 17 Oct 2020 19:27:08 +0200
+Subject: [PATCH] boost: Do not add system paths on nix
+
+---
+ mesonbuild/dependencies/boost.py | 17 +----------------
+ 1 file changed, 1 insertion(+), 16 deletions(-)
+
+diff --git a/mesonbuild/dependencies/boost.py b/mesonbuild/dependencies/boost.py
+index 907c0c275..ecaf11b18 100644
+--- a/mesonbuild/dependencies/boost.py
++++ b/mesonbuild/dependencies/boost.py
+@@ -643,22 +643,7 @@ class BoostDependency(ExternalDependency):
+             roots += [x for x in candidates if x.name.lower().startswith('boost') and x.is_dir()]
+         else:
+             tmp = []  # type: T.List[Path]
+-
+-            # Homebrew
+-            brew_boost = Path('/usr/local/Cellar/boost')
+-            if brew_boost.is_dir():
+-                tmp += [x for x in brew_boost.iterdir()]
+-
+-            # Add some default system paths
+-            tmp += [Path('/opt/local')]
+-            tmp += [Path('/usr/local/opt/boost')]
+-            tmp += [Path('/usr/local')]
+-            tmp += [Path('/usr')]
+-
+-            # Cleanup paths
+-            tmp = [x for x in tmp if x.is_dir()]
+-            tmp = [x.resolve() for x in tmp]
+-            roots += tmp
++            # Do not add any non-explicit paths on nix
+ 
+         return roots
+ 
+-- 
+2.25.4
+

pkgs/development/tools/build-managers/meson/default.nix

@@ -52,6 +52,11 @@ python3.pkgs.buildPythonApplication rec {
     # cut-in-half-by-\0 store path references.
     # Let’s just clear the whole rpath and hope for the best.
     ./clear-old-rpath.patch
+
+    # Patch out default boost search paths to avoid impure builds on
+    # unsandboxed non-NixOS builds, see:
+    # https://github.com/NixOS/nixpkgs/issues/86131#issuecomment-711051774
+    ./boost-Do-not-add-system-paths-on-nix.patch
   ];
 
   setupHook = ./setup-hook.sh;