Add an Auto option to live_compile

Ass suggested by @zeroSteiner

lib/msf/core/post/linux/compile.rb

@@ -11,7 +11,7 @@ def initialize(info = {})
     super
     register_options( [
       OptEnum.new('COMPILE', [true, 'Compile on target', 'Auto', ['Auto', 'True', 'False']]),
-      OptEnum.new('COMPILER', [true, 'Compiler to use on target', 'gcc', ['gcc', 'clang']]),
+      OptEnum.new('COMPILER', [true, 'Compiler to use on target', 'Auto', ['Auto', 'gcc', 'clang']]),
     ], self.class)
   end
 
@@ -36,6 +36,16 @@ def live_compile?
   def upload_and_compile(path, data, compiler_args='')
     write_file "#{path}.c", strip_comments(data)
 
+    if datastore['COMPILER'] == 'Auto'
+      if has_gcc?
+        datastore['COMPILER'] = 'gcc'
+      elsif has_clang?
+        datastore['COMPILER'] = 'clang'
+      else
+        fail_with Module::Failure::BadConfig, "Unable to find gcc nor clang on the remote target."
+      end
+    end
+
     compiler_cmd = "#{datastore['COMPILER']} -o '#{path}' '#{path}.c'"
     if session.type == 'shell'
       compiler_cmd = "PATH=\"$PATH:/usr/bin/\" #{compiler_cmd}"