autogen.sh should exit with non-zero if one of its commands fails

[vivanishin]

Right now, if, say configure fails, autogen.sh would continue execution and exit with the exit code of the last command. (Caught this on a box with missing libreadline.)

This is rarely needed, because distributions normally use tarballs (and thus skip autogen.sh altogether), but it affects the scripting of building from git.

[rocky]

Feel free to put in a PR for this.