diagnostic: fix autoconf check

Xcode can only provide autotools if it is installed, thus check that
first. Skipping this check will try to compare a `nil` Xcode version to
4.3, the first version of Xcode to not provide autotools.

Fixes Homebrew/homebrew#48208.

Closes Homebrew/homebrew#48278.

Signed-off-by: Martin Afanasjew <martin@afanasjew.de>

Library/Homebrew/diagnostic.rb

@@ -1044,6 +1044,7 @@ module Homebrew
       end
 
       def check_for_autoconf
+        return unless MacOS::Xcode.installed?
         return unless MacOS::Xcode.provides_autotools?
 
         autoconf = which("autoconf")

Library/Homebrew/test/test_diagnostic.rb

@@ -205,6 +205,7 @@ class DiagnosticChecksTest < Homebrew::TestCase
   end
 
   def test_check_for_autoconf
+    MacOS::Xcode.stubs(:installed?).returns true
     MacOS::Xcode.stubs(:provides_autotools?).returns true
     mktmpdir do |path|
       file = "#{path}/autoconf"