Make automake 1.14 shut-up about subdir-objects

使用automake 1.14，发现老项目在编译过程中会报警告：

source/Makefile.am:45: warning: source file 'zz/xx.cpp' is in a subdirectory,
source/Makefile.am:45: but option 'subdir-objects' is disabled

解决方法
在configure.ac中添加如下内容：

AM_INIT_AUTOMAKE([subdir-objects])