Ability to run nixpkgs on FreeBSD host

[itzwam]

When I dont force the MAKEOBJDIRPREFIX var to /
I can't build most of the system libraries when running on a FreeBSD host (w/ Lix compiled from srcs)
most of the lib files are built in a place where linker cannot find it,

https://github.com/lattera/freebsd/blob/master/share/mk/bsd.obj.mk#L56-L63
by exporting MAKEOBJDIRPREFIX=/ compilations will now produce all obj outputs in source directory, linker then finds them well 👍

With this issue fixed I'm able to launch most basic nix-shells in a FreeBSD host using nixbsd pkgs fork 🥳

[itzwam]

diff --git a/pkgs/stdenv/generic/default.nix b/pkgs/stdenv/generic/default.nix
index 2cda43d5632f..c8d9c232a26a 100644
--- a/pkgs/stdenv/generic/default.nix
+++ b/pkgs/stdenv/generic/default.nix
@@ -114,6 +114,8 @@ let
         export NIX_NO_SELF_RPATH=1
       '' + lib.optionalString (hostPlatform.isDarwin && hostPlatform.isMacOS) ''
         export MACOSX_DEPLOYMENT_TARGET=${hostPlatform.darwinMinVersion}
+      '' + lib.optionalString (hostPlatform.isFreeBSD) ''
+        export MAKEOBJDIRPREFIX=/
       ''
       # TODO this should be uncommented, but it causes stupid mass rebuilds. I
       # think the best solution would just be to fixup linux RPATHs so we don't