Fix compilation with OpenMP on latest macOS Xcode without headers in /usr/include

[fox34]

When installing according to the docs on macOS 10.14.5, compilation with OpenMP support fails on newer macOS/Xcode versions.

Latest versions of Xcode do not provide the mentioned package macOS_SDK_headers_for_macOS_10.14.pkg any more, as documented in the Xcode 10 release notes.

As such, /usr/include does not exist and cannot be created. Thus, the compilation always fails with the error stdio.h file not found.

Modifying the last line of Makevars from the recommended

CPPFLAGS=-I/usr/local/opt/gettext/include -I$(LLVM_LOC)/include

to

CPPFLAGS=-I/usr/local/opt/gettext/include -I$(LLVM_LOC)/include -I/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include

solved the issue for me.
However, a lot of warnings and notes about nullability were issued during compilation, but I do not know whether this is associated with my fix or not.

[mattdowle]

Many thanks. I've added this with a link back to this issue, to https://github.com/Rdatatable/data.table/wiki/Installation.