6. Writing `configure.in'

Writing a portable `configure.in' is a tricky business. Since you can put arbitrary shell code into `configure.in', your options seem overwhelming. There are many questions the first-time Autoconf user asks: What constructs are portable and what constructs aren't portable? How do I decide what to check for? What shouldn't I check for? How do I best use Autoconf's features? What shouldn't I put in `configure.in'? In what order should I run my checks? When should I look at the name of the system instead of checking for specific features?