Xcode-only: Clean up compiler flags. Use CPATH.
Xcode-only: Clean up compiler flags. Use CPATH.

- When using CPATH, we only need -isysroot and not the other -I spam.
- LDFLAGS use -isysroot instead of -L
  Turns out that the linker also understands the
  -isysroot flag which behaves more natural (in the
  sense: "like the internal defaults `/usr/lib` etc.)
  than adding `-L$(SDKROOT).
  Especially for duplicates, the linker first searches
  through all `-L` dirs and then looks at the sysroot
  or internal defaults. This is what we want.
- Pkg-config needs CC with CFLAGS passed to configure.

Closes Homebrew/homebrew#14351.

Signed-off-by: Jack Nagel <43386ce32af96f5c56f2a88e458cb94cebee3751@gmail.com>

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