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: NEugene (jno) Dvurechenski <jno@linux.vnet.ibm.com>
Suggested-by: NMarkus Armbruster <armbru@redhat.com>
Signed-off-by: NCornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

configure

@@ -3,6 +3,11 @@
 # qemu configure script (c) 2003 Fabrice Bellard
 #
 
+# Unset some variables known to interfere with behavior of common tools,
+# just as autoconf does.
+CLICOLOR_FORCE= GREP_OPTIONS=
+unset CLICOLOR_FORCE GREP_OPTIONS
+
 # Temporary directory used for files created while
 # configure runs. Since it is in the build directory
 # we can safely blow away any previous version of it