Fix install on mojave

[convenient]

There were two header issues which prevented PHP being compiled on the latest OSX mojave.

Firstly apple changed how their headers were handled. (emphasis mine)

https://developer.apple.com/documentation/xcode_release_notes/xcode_10_release_notes

The command line tools will search the SDK for system headers by default. However, some software may fail to build correctly against the SDK and require macOS headers to be installed in the base system under /usr/include. If you are the maintainer of such software, we encourage you to update your project to work with the SDK or file a bug report for issues that are preventing you from doing so. As a workaround, an extra package is provided which will install the headers to the base system. In a future release, this package will no longer be provided. You can find this package at:

/Library/Developer/CommandLineTools/Packages/macOS_SDK_headers_for_macOS_10.14.pkg

Secondly there was a bug in the PHP src with how it handled includes itself.

php/php-src#3472

This PR is largely a port of what was done in the homebrew-core here Homebrew/homebrew-core@edf393e