cli/build.rs requires glibc on Linux

[mdekstrand]

Currently build.rs hard-depends on being able to load glibc in order to set linker flags (Line 430):

 #[cfg(target_os = "linux")]
{
  let ver = glibc_version::get_version().unwrap();

  // If a custom compiler is set, the glibc version is not reliable.
  // Here, we assume that if a custom compiler is used, that it will be modern enough to support a dynamic symbol list.
  if env::var("CC").is_err() && ver.major <= 2 && ver.minor < 35 {
    println!("cargo:warning=Compiling with all symbols exported, this will result in a larger binary. Please use glibc 2.35 or later for an optimised build.");
    println!("cargo:rustc-link-arg-bin=deno=-rdynamic");
  } else {
    println!(
      "cargo:rustc-link-arg-bin=deno=-Wl,--export-dynamic-symbol-list={}",
      symbols_path.display()
    );
  }
}

Correcting this is the only source code change I needed to successfully build Deno on Alpine/musl x86_64 and will help address #3711. I would submit a PR directly but am not sure what the best fix here is; the failure is that unwrap() fails, because glibc can't be loaded; in my build I removed all the conditionals and the glibc call here so that it just uses the dynamic symbol list without checking.

Maybe if glibc is missing assume it's good enough? Or should MUSL-building code set CC? That will still require a source edit, because currently glibc is checked before CC. This whole block could also be made dependent on the target also being GNU.