make: Use the MacOSX 10.13 SDK for x86 support.

make: Use the MacOSX 10.13 SDK for x86 support.

Bugfix - CFLAGS += instead of CFLAGS = because it would get overwritten otherwise.

Also allow overwriting of XCODE_PATH for non-standard path.

Download the 10.13 SDK from
https://github.com/phracker/MacOSX-SDKs/releases

```bash
wget https://github.com/phracker/MacOSX-SDKs/releases/download/10.13/MacOSX10.13.sdk.tar.xz
xz --uncompress MacOSX10.13.sdk.tar.xz
tar xvf MacOSX10.13.sdk.tar
mv MacOSX10.13.sdk /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/
```