system_config: fix clang version output on Linux

If you have Homebrew LLVM installed, `brew config` will show

    Clang: 12.0.1 build (parse error)

because Homebrew LLVM does not give a build number. Some installations
from the system package manager do have this information, so let's show
it if it's available but skip complaining if not.

I've kept the original code for macOS because we always expect Apple's
build number to show there.

Library/Homebrew/extend/os/mac/system_config.rb

@@ -7,7 +7,7 @@ module SystemConfig
   class << self
     include SystemCommand::Mixin
 
-    undef describe_homebrew_ruby
+    undef describe_homebrew_ruby, describe_clang
 
     def describe_homebrew_ruby
       s = describe_homebrew_ruby_version
@@ -19,6 +19,13 @@ module SystemConfig
       end
     end
 
+    def describe_clang
+      return "N/A" if clang.null?
+
+      clang_build_info = clang_build.null? ? "(parse error)" : clang_build
+      "#{clang} build #{clang_build_info}"
+    end
+
     def xcode
       @xcode ||= if MacOS::Xcode.installed?
         xcode = MacOS::Xcode.version.to_s

Library/Homebrew/system_config.rb

@@ -76,8 +76,11 @@ module SystemConfig
     def describe_clang
       return "N/A" if clang.null?
 
-      clang_build_info = clang_build.null? ? "(parse error)" : clang_build
-      "#{clang} build #{clang_build_info}"
+      if clang_build.null?
+        clang.to_s
+      else
+        "#{clang} build #{clang_build}"
+      end
     end
 
     def describe_path(path)