perl 5.8.7: Use grep -I to ignore binary files instead of looking at

perl 5.8.7: Use grep -I to ignore binary files instead of looking at
the output message from grep. The output message could change
depending on the LOCALE which would break the current test. Make it
DEPEND on grep-native to ensure we get the gnu version of grep which
is needed for -I to work. As per bug #1439.