Error when find-llvm-config is unsuccessful

[straight-shoota]

If find-llvm-config can't find an LLVM installation, it silent about it, print nothing and exiting as success. This results in undecipherable errors later on when LibLLVM::LLVM_CONFIG is called.

This patch changes the helper to error with status 1 and print an informative message to stderr if LLVM_CONFIG was not found.

Before:

$ bin/crystal spec spec/std/llvm/
--: line 0: : Permission denied
Showing last frame. Use --error-trace for full trace.

In src/llvm/lib_llvm.cr:13:17

 13 | VERSION = {{`#{LibLLVM::LLVM_CONFIG} --version`.chomp.stringify}}
                  ^
Error: error executing command: "" --version, got exit status 127

After:

$ bin/crystal spec spec/std/llvm/
Error: Could not find location of llvm-config. Please specify path in environment variable LLVM_CONFIG.
Supported LLVM versions: 14 13 12 11.1 11 10 9 8
Showing last frame. Use --error-trace for full trace.

In src/llvm/lib_llvm.cr:3:42

 3 | LLVM_CONFIG = {{ env("LLVM_CONFIG") || `#{__DIR__}/ext/find-llvm-config`.stringify }}
                                            ^
Error: error executing command: /app/src/llvm/ext/find-llvm-config, got exit status 1

Resolves https://forum.crystal-lang.org/t/problem-compiling-crystalline-for-the-vscode-plugin/5316

[sdogruyol]

Thank you @straight-shoota 🙏

[straight-shoota]

I added another line to print the supported LLVM versions.