Set 'makeprg' variable as 'make' when Makefile exists

vim-go will change 'makeprg' variable to 'go build' by default,
but it's not what we want when user has created his own Makefile
for golang project.

Let's set 'makeprg' variable as 'make' when Makefile exists in
current directory.

Signed-off-by: Yunkai Zhang <qiushu.zyk@taobao.com>