Xcode 11.4 has an updated sys/_types/_fd_def.h header that references a
new symbol from libSystem. This is a problem because we're using
`/usr/bin/clang` to compile the non-Xcode portion, and this pulls in
headers from Xcode's SDK. Somehow it's still linking to the Nix
libraries (I can't figure out where configure finds these to put into
`LDFLAGS` as we're not using the cc-wrapper). The end result is we get a
linker error where this new symbol can't be found at link time, even
though it's a weak import and isn't required at runtime.

Ideally we'd provide a full 10.12 SDK to `/usr/bin/clang`, but we can't
do that because even the DevSDK package we use for our 10.12 SDK doesn't
contain everything (in particular it's missing nearly all dylibs) so we
just get linker errors if we do that.

Instead we'll just do a horrible hack and provide an `-isystem` path to
a folder structure that contains only the 10.12 `sys/_types/_fd_def.h`
header. This avoids the new symbol without causing all the errors that
happen if we pull in the entire `${darwin.Libsystem}/include`.

MacVim compiles the Vim part using `/usr/bin/clang` and the GUI part
using Xcode. The Xcode portion always uses Xcode's own SDK and we have
no workable alternative. The Vim portion so far has been compiling using
a hybrid compilation environment, where it uses the SDK for most stuff
but picks up a bunch of library linker paths (including libSystem) by
virtue of Ruby's LDFLAGS. This hybrid compilation environment meant that
if the SDK headers referenced a symbol that the library itself didn't
have, this could produce link errors.

Previously we attempted to fix this by synthesizing an include path that
contained just the one header from Nix's Libsystem that referenced the
missing symbol, to get rid of the reference and allow linking to work
again, but this was very hacky and runs the risk of future Xcode SDK
changes producing the same errors with different headers, or of future
SDK versions expecting the intercepted header to contain a definition
that Nix's doesn't.

This new approach is to just clean up the compilation environment such
that the Vim portion is compiling against the Xcode SDK as well, by
sanitizing the LDFLAGS produced by the configure script so it stops
referencing Nix's versions of OS libraries. This means the resulting Vim
binary no longer depends at runtime on Nix for anything except the
scripting language support, but that's how it's been for the MacVim
binary all along anyway, and this approach should keep us insulated
against future Xcode SDK changes.