diffstat: fix wrong evaluation of CC

diffstat: fix wrong evaluation of CC

diffstat checks variable CC. If options '-I', '-U' or '-D' is included
in CC, variable CC will be wrongly filtered and only such as
'mips64-wrsmllibn32-linux-gnun32-gcc' left. It shows warning message
during configure:

| checking $CC variable...broken
| configure: WARNING: your environment misuses the CC variable to hold CFLAGS/CPPFLAGS options

Disable such check for OE.

Signed-off-by: Kai Kang <kai.kang@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>