use xcrun to run Apple clang

[tizee]

I've installed LLVM's clang on my machine and it has higher priority than Apple's clang in shell's PATH. When I run make it would produce following errors:

clang src/skhd.c -std=c99 -Wall -g -O0 -framework Cocoa -framework Carbon -framework CoreServices -o bin/skhd
In file included from src/skhd.c:30:
src/service.h:136:9: error: call to undeclared function '_NSGetExecutablePath'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
    if (_NSGetExecutablePath(exe_path, &exe_path_size) < 0) {

After I force it to use Apple's clang then the error is gone.