bindgen panics with "header not found" message if Clang version is too old

bindgen crate doesn't clearly states Clang version is too old. Build fails with <header> not found error. See iai-callgrind issue.

Steps to reproduce:

1. On Ubuntu 18.04 install libclang-6.0-dev, remove all other libclang versions if any
2. Follow tutorial to create a sample project
3. Build project using cargo build

Expected result: error message about too old Clang version

Actual result: 'stddef.h' file not found error:

# cargo build
   Compiling t v0.1.0 (/root/t)
error: failed to run custom build command for `t v0.1.0 (/root/t)`

Caused by:
  process didn't exit successfully: `/root/t/target/debug/build/t-fc2313a9f8a632f7/build-script-build` (exit status: 101)
  --- stdout
  cargo:rustc-link-search=/path/to/lib
  cargo:rustc-link-lib=bz2
  cargo:rerun-if-env-changed=TARGET
  cargo:rerun-if-env-changed=BINDGEN_EXTRA_CLANG_ARGS_x86_64-unknown-linux-gnu
  cargo:rerun-if-env-changed=BINDGEN_EXTRA_CLANG_ARGS_x86_64_unknown_linux_gnu
  cargo:rerun-if-env-changed=BINDGEN_EXTRA_CLANG_ARGS
  cargo:rerun-if-changed=wrapper.h

  --- stderr
  /usr/include/stdio.h:33:10: fatal error: 'stddef.h' file not found

  thread 'main' panicked at build.rs:25:10:
  Unable to generate bindings: ClangDiagnostic("/usr/include/stdio.h:33:10: fatal error: 'stddef.h' file not found\n")
  note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Upgrading Clang to version 9 solves the issue.

Reproduce the failure with Ubuntu 18.04 and libclang-6.0-dev, following the tutorial and using the reported wrapper.h and build.rs:25 context. Inspect how the build currently reports Clang diagnostics; done when an unsupported old Clang installation produces a clear version-related error instead of only reporting that stddef.h is missing.