[1] GNU Autoconf, Automake and Libtool,
by G. V. Vaughan, B. Elliston, T. Tromey, and I. L. Taylor. SAMS (originally
New Riders), 2000, ISBN 1578701902.
[2] Because M4 is not aware of Sh code,
especially conditionals, some optimizations that look nice statically
may produce incorrect results at runtime.
