Unused $CPP is shown as a configure option

In configure, we were calling AC_PROG_CPP but never making use of the $CPP variable it sets or reads.

The issue is $CPP will show up in the --help output of configure, falsely advertising a configuration option that does nothing.

The reason we don't use the $CPP variable is because HS_CPP_CMD is expected to be a single command (without flags), but AC_PROG_CPP, when CPP is unset, will set said variable to something like /usr/bin/gcc -E. Instead, we configure HS_CPP_CMD through $CC.

Fixed in !10431 (closed)