How to get to C <stdarg.h> and friends elegantly ?

What I am looking for is a way to specify the include directory for (C not C++) stdarg.h, which for Xcode 7.0 apparently is

/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/clang/7.0.0/include

The settting

$(TOOLCHAIN_DIR)/usr/lib/clang/`clang --version | head -1 | sed 's/Apple LLVM version \([^ ]*\).*/\1/'`/include

is not very elegant, and also doesn't work as an Xcode setting definition. Manually setting the version number for clang everytime seems painful. This comes up, when you want to use a self-compiled clang, that has it's own xcspec.