configure: unset interfering variables

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 <[email protected]>
Suggested-by: Markus Armbruster <[email protected]>
Signed-off-by: Cornelia Huck <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>