freebsd.libthr: don't leave dangling symlink in static configuration

pkgs/os-specific/bsd/freebsd/pkgs/libthr.nix

@@ -1,4 +1,6 @@
 {
+  lib,
+  stdenv,
   mkDerivation,
   libcMinimal,
   include,
@@ -33,5 +35,9 @@ mkDerivation {
     export NIX_CFLAGS_COMPILE="$NIX_CFLAGS_COMPILE -B${csu}/lib"
   '';
 
+  postInstall = lib.optionalString stdenv.hostPlatform.isStatic ''
+    rm $out/lib/libpthread.so
+  '';
+
   env.MK_TESTS = "no";
 }