Support variable declaration initializer

rakelib/boot.rake

@@ -17,7 +17,7 @@ def gcc_verbose(gcc_binary)
 end
 
 def find_build_target
-  gcc_verbose('gcc')
+  gcc_verbose(ENV.fetch 'CC', 'gcc')
     .lines
     .find { |line| line.start_with? 'Target: ' }
     .split(' ')