configure: unset interfering variables

The check for big or little endianness relies on grep reporting
match/non-match on the generated binary. If the user specified
--binary-files=without-match in their GREP_OPTIONS, this will fail.

Let's follow what autoconf does and unset GREP_OPTIONS and CLICOLOR_FORCE
at the beginning of the script.

Reported-by: Eugene (jno) Dvurechenski <jno@linux.vnet.ibm.com>
Suggested-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
