superenv cc: accept single-digit g++ version

Closes Homebrew/homebrew#40117.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Library/ENV/4.3/cc

@@ -44,13 +44,13 @@ class Cmd
     @tool ||= case @arg0
     when 'ld' then 'ld'
     when 'cpp' then 'cpp'
-    when /\w\+\+(-\d\.\d)?$/
+    when /\w\+\+(-\d(\.\d)?)?$/
       case ENV['HOMEBREW_CC']
       when /clang/
         'clang++'
       when /llvm-gcc/
         'llvm-g++-4.2'
-      when /gcc(-\d\.\d)?$/
+      when /gcc(-\d(\.\d)?)?$/
         'g++' + $1.to_s
       end
     else