ENV: Ensure @compiler is set

If @compiler isn't set/checked by ENV.compiler and the ENV.clang/etc.
methods, ENV.compiler may return unexpected results after fails_with
picks a fallback compiler.

See Homebrew/homebrew#22674.

Library/Homebrew/extend/ENV/shared.rb

@@ -82,7 +82,7 @@ module SharedEnvExtension
   def fcflags;  self['FCFLAGS'];      end
 
   def compiler
-    if (cc = ARGV.cc)
+    @compiler ||= if (cc = ARGV.cc)
       COMPILER_SYMBOL_MAP.fetch(cc) do |other|
         if other =~ GNU_GCC_REGEXP then other
         else

Library/Homebrew/extend/ENV/super.rb

@@ -263,16 +263,19 @@ module Superenv
   alias_method :j1, :deparallelize
   def gcc
     self['HOMEBREW_CC'] = "gcc-4.2"
+    @compiler = :gcc
   end
   def llvm
     self['HOMEBREW_CC'] = "llvm-gcc"
+    @compiler = :llvm
   end
   def clang
     self['HOMEBREW_CC'] = "clang"
+    @compiler = :clang
   end
   GNU_GCC_VERSIONS.each do |n|
     define_method(:"gcc-4.#{n}") do
-      self['HOMEBREW_CC'] = "gcc-4.#{n}"
+      @compiler = self['HOMEBREW_CC'] = "gcc-4.#{n}"
     end
   end
   def make_jobs