Don't reject -L/usr/local/lib when SDKROOT is defined

[tdsmith]

/usr/local/lib is removed from the default linker search path when
SDKROOT is defined or sysroot is specified.

Homebrew sometimes sets SDKROOT without setting a sysroot as of 45e138f.

This is the simplest possible change, but there are some other options:

• Modify effective_sysroot so that HOMEBREW_SDKROOT gets set identically; maybe @ilovezfs knows why this wasn't done originally?
• Add an additional accessor for SDKROOT
• ?

Fixes #844.

[MikeMcQuaid]

Seems reasonable to me but I don't see any reference to the prefix in this or the referenced commit but this seems to only relate to a non-standard prefix, right? It feels like this is something that should vary depending on the prefix as /usr/local/lib is a system prefix now but we want it filtered if that's the HOMEBREW_PREFIX.

[tdsmith]

It looks like that case is handled elsewhere; refurbish_args rejects linker search paths that are present in library_paths or system_library_paths and HOMEBREW_PREFIX/lib is added to library_paths by superenv.

[MikeMcQuaid]

refurbish_args rejects linker search paths that are present in library_paths or system_library_paths and HOMEBREW_PREFIX/lib is added to library_paths by superenv.

👍

[ilovezfs]

I'm still not clear why /usr/local is being hard coded here. By default there's nothing there, so how is it a "special" path?

[MikeMcQuaid]

@ilovezfs Probably because it's in the default search path for Clang.

[ilovezfs]

Ah ok