Improve check for openssl on macOS

- Only call `openssl version` once;

- Make the check work with future version of LibreSSL after 2.2.7. This
  assumes that Apple might keep updating LibreSSL shipping with macOS,
  but that they will never provide header files.

bin/ruby-build

@@ -958,11 +958,9 @@ use_homebrew_readline() {
 }
 
 has_broken_mac_openssl() {
-  is_mac &&
-  [[
-    "$(/usr/bin/openssl version 2>/dev/null || true)" = "OpenSSL 0.9.8"?* ||
-    "$(/usr/bin/openssl version 2>/dev/null || true)" = "LibreSSL 2.2.7"
-  ]] &&
+  is_mac || return 1
+  local openssl_version="$(/usr/bin/openssl version 2>/dev/null || true)"
+  [[ $openssl_version = "OpenSSL 0.9.8"?* || $openssl_version = "LibreSSL"* ]] &&
   [[ "$RUBY_CONFIGURE_OPTS" != *--with-openssl-dir=* ]] &&
   ! use_homebrew_openssl
 }