Add audit for xcodebuild.

xcodebuild should be passed an explicit SYMROOT, since the user
can override this.

Also added --strict, so certain audits can be ignored in most
cases (including this one.)

Library/Homebrew/cmd/audit.rb

@@ -1,6 +1,12 @@
 require 'formula'
 require 'utils'
 
+# Use "brew audit --strict" to enable even stricter checks.
+
+def strict?
+  ARGV.flag? "--strict"
+end
+
 def ff
   return Formula.all if ARGV.named.empty?
   return ARGV.formulae
@@ -103,6 +109,11 @@ def audit_formula_text name, text
     problems << " * Use ENV.fortran during install instead of depends_on 'gfortran'"
   end unless name == "gfortran" # Gfortran itself has this text in the caveats
 
+  # xcodebuild should specify SYMROOT
+  if text =~ /xcodebuild/ and not text =~ /SYMROOT=/
+    problems << " * xcodebuild should be passed an explicit \"SYMROOT\""
+  end if strict?
+
   return problems
 end
 
@@ -182,7 +193,7 @@ def audit_formula_urls f
 
     unless p =~ %r[^http://mirrors\.kernel\.org/debian/pool/]
       problems << " * \"mirrors.kernel.org\" is the preferred mirror for debian software."
-    end
+    end if strict?
   end
 
   return problems