Remove --with-CC autoconf option

[Heikki Linnakangas]

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])

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