RFA: Fix problem with trailing slashes in include paths on mingw

[Zack Weinberg]

Joern RENNECKE <joern.rennecke@st.com> writes:

> Zack Weinberg wrote:
>
>>Oops.  Okay, in that case
>>
>>  while (c[-1] == '/' && c > path+1)
>
> c might be equal to path.

Unlikely (it could only happen with -I "", and I think that gets
canonicalized to -I . earlier), but go ahead and reverse the order of
the tests.

zw