boost : extra arguments to b2

Certain boost features require compile time flags. For example -
https://www.boost.org/doc/libs/1_71_0/libs/test/doc/html/boost_test/adv_scenarios/obsolete_init_func.html
https://www.boost.org/doc/libs/1_71_0/libs/test/doc/html/boost_test/utf_reference/link_references/link_boost_test_no_main.html
https://www.boost.org/doc/libs/1_71_0/libs/test/doc/html/boost_test/utf_reference/link_references/link_boost_test_dyn_link.html

This commit add the ability to specify arbitrary extra arguments to b2's arguments.

pkgs/development/libraries/boost/generic.nix

@@ -14,6 +14,7 @@
 , taggedLayout ? ((enableRelease && enableDebug) || (enableSingleThreaded && enableMultiThreaded) || (enableShared && enableStatic))
 , patches ? []
 , mpi ? null
+, extraB2Args ? []
 
 # Attributes inherit from specific versions
 , version, src
@@ -92,7 +93,8 @@ let
     ++ optional (mpi != null || stdenv.hostPlatform != stdenv.buildPlatform) "--user-config=user-config.jam"
     ++ optionals (stdenv.hostPlatform.libc == "msvcrt") [
     "threadapi=win32"
-  ]);
+  ] ++ extraB2Args
+  );
 
 in