shared: refactor gcc_version_formula.

Avoid throwing unnecessary exceptions by checking for paths existing and
creating formulae as late as possible. Additionally use instance
variables for some caching.

Library/Homebrew/extend/ENV/shared.rb

@@ -184,24 +184,30 @@ module SharedEnvExtension
   end
 
   def gcc_version_formula(version)
-    gcc_formula = Formulary.factory("gcc")
-    gcc_name = 'gcc' + version.delete('.')
+    gcc_name = "gcc-#{version}"
+    gcc_version_name = "gcc#{version.delete('.')}"
 
-    if gcc_formula.opt_prefix.exist?
-      return gcc_formula
-    end
+    ivar = "@#{gcc_version_name}_version"
+    return instance_variable_get(ivar) if instance_variable_defined?(ivar)
 
-    gcc_versions_formula = Formulary.factory(gcc_name) rescue nil
+    gcc_path = HOMEBREW_PREFIX.join "opt/gcc/bin/#{gcc_name}"
+    gcc_formula = Formulary.factory "gcc"
+    gcc_versions_path = \
+      HOMEBREW_PREFIX.join "opt/#{gcc_version_name}/bin/#{gcc_name}"
 
-    if gcc_versions_formula && gcc_versions_formula.opt_prefix.exist?
-      gcc_versions_formula
+    formula = if gcc_path.exist?
+      gcc_formula
+    elsif gcc_versions_path.exist?
+      Formulary.factory gcc_versions_formula
     elsif gcc_formula.version.to_s.include?(version)
       gcc_formula
-    elsif gcc_versions_formula
+    elsif (gcc_versions_formula = Formulary.factory(gcc_version_name) rescue nil)
       gcc_versions_formula
     else
-      Formulary.factory(gcc_name)
+      gcc_formula
     end
+
+    instance_variable_set(ivar, formula)
   end
 
   def warn_about_non_apple_gcc(gcc)