Add llvm-config --cflags to CFLAGS (#59419)
It's convenient to use `USE_SYSTEM_LLVM=1` and `LLVM_CONFIG` to build
Julia with a local LLVM build, but the build fails because
`<llvm/Config/llvm-config.h>` is not found when it is not installed into
`build_includedir`. Ask `llvm-config` for the `CFLAGS` to fix this.
