unix: default CC in mkerrors.sh to cc not gcc

Clang is much more common now so use the OS cc instead of forcing gcc
by default.

Change-Id: I1dd34c7ba56c91794ee57042dd8ac024fdd649d8
Reviewed-on: https://go-review.googlesource.com/15010
Reviewed-by: Rob Pike <r@golang.org>
1 file changed