Require autoconf 2.61 as a minimum, and eliminate stuff that does for you.

Require autoconf 2.61 as a minimum, and eliminate stuff that does for you.

autoconf 2.61 and later do, in AC_PROG_CC, tests to figure out how to
beat non-GCC compilers into doing the closest thing to ANSI C of which
they're capable, so we don't need to do them ourselves.