Fix some BSD builds missing pthread functions

In addition to adding the missing thread deps in the last commit, we
also appear to need to skip `-Wl,--as-needed` flags that Meson wants to
use, but doesn't work with our *BSD toolchains.

See mesonbuild/meson#3593

packaging/dependencies.nix

@@ -61,6 +61,12 @@ let
       workDir = null;
     };
 
+  # Work around weird `--as-needed` linker behavior with BSD, see
+  # https://github.com/mesonbuild/meson/issues/3593
+  bsdNoLinkAsNeeded = finalAttrs: prevAttrs: lib.optionalAttrs stdenv.hostPlatform.isBSD {
+    mesonFlags = [ (lib.mesonBool "b_asneeded" false) ] ++ prevAttrs.mesonFlags or [];
+  };
+
 in
 scope: {
   inherit stdenv versionSuffix;
@@ -130,5 +136,8 @@ scope: {
 
   inherit resolvePath filesetToSource;
 
-  mkMesonDerivation = f: stdenv.mkDerivation (lib.extends localSourceLayer f);
+  mkMesonDerivation = f: stdenv.mkDerivation
+    (lib.extends
+      (lib.composeExtensions bsdNoLinkAsNeeded localSourceLayer)
+      f);
 }