Fix build warnings for mruby-sys

[lopopolo]

$ cargo clippy --all-targets --all-features
warning: couldn't execute `llvm-config --prefix` (error: No such file or directory (os error 2))
warning: set the LLVM_CONFIG_PATH environment variable to a valid `llvm-config` executable
    Finished dev [unoptimized + debuginfo] target(s) in 0.50s

I think this was caused by switching the mruby build toolchain to clang.

artichoke/mruby-sys/debug_build_config.rb

Line 10 in ff42613

toolchain :clang

I'm getting these warnings on macOS 10.14.6 with this clang

$ clang --version
Apple LLVM version 10.0.1 (clang-1001.0.46.4)
Target: x86_64-apple-darwin18.7.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin

[abelsromero]

Also cargo build, cargo test, etc. fail on Linux if clang is not installed.
This should probably be explained in the toolchain guide: https://github.com/artichoke/artichoke/blob/master/CONTRIBUTING.md#rust-toolchain

[lopopolo]

Building mruby-sys currently depends on a clang toolchain being installed + bintools + yacc + gperf + ruby.

artichoke/.circleci/config.yml

Lines 48 to 61 in 6ad780b

	- run:
	# https://github.com/mruby/mruby/blob/master/doc/guides/compile.md#prerequisites
	name: Install mruby Build Dependencies
	command: |
	sudo apt-get install -y binutils bison gperf ruby-full
	bison --version
	gperf --version
	ruby --version
	- run:
	# needed for cc crate in build.rs
	name: Install mruby-sys Build Dependencies
	command: |
	sudo apt-get install -y clang
	clang --version

I have ideas on how we can modify the mruby-sys build to depend on the cc crate. This would still depend on yacc for generating the parser C source and maybe gperf.

See GH-145.

[lopopolo]

See artichoke/artichoke-ci#4

[lopopolo]

These warnings come from clang-sys which is pulled in by bindgen. These warnings will go away once bindgen releases a version that depends on clang-sys 1.0.0.

rust-lang/rust-bindgen#1826 (comment)

[lopopolo]

Fixed by #804