Remove --with-CC autoconf option

Any objections to removing the ./configure --with-CC option? It's been
deprecated since commit cb292206c5 from July 2000:

> # For historical reasons you can also use --with-CC to specify the C compiler
> # to use, although the standard way to do this is to set the CC environment
> # variable.
> PGAC_ARG_REQ(with, CC, [CMD], [set compiler (deprecated)], [CC=$with_CC])

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Heikki Linnakangas
Neon (https://neon.tech)