boost: refine assertion requiring pth support in clang (#159990)

The pth feature was only used by boost's build with clang on linux,
and should not prevent building on darwin.

pkgs/development/libraries/boost/generic.nix

@@ -31,8 +31,8 @@ assert enableShared || enableStatic;
 assert enablePython -> stdenv.hostPlatform == stdenv.buildPlatform;
 assert enableNumpy -> enablePython;
 
-# Boost <1.69 can't be build with clang >8, because pth was removed
-assert with lib; ((toolset == "clang" && !(versionOlder stdenv.cc.version "8.0.0")) -> !(versionOlder version "1.69"));
+# Boost <1.69 can't be built on linux with clang >8, because pth was removed
+assert with lib; ((stdenv.isLinux && toolset == "clang" && !(versionOlder stdenv.cc.version "8.0.0")) -> !(versionOlder version "1.69"));
 
 with lib;
 let