configure: unset harmful environment variables

configure: unset harmful environment variables

Apart from CLICOLOR_FORCE and GREP_OPTIONS, there are other variables
that are listed in the Autoconf manual.  While Autoconf neutralizes them
very early, and assumes it does not (yet) run in a shell that has "unset",
QEMU assumes that the user invoked configure under a POSIX shell, and
therefore can simply use "unset" to clear them.

CDPATH is particularly nasty because it messes up "cd ... && pwd".

Reported-by: Juan Quintela <quintela@redhat.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>