[vcpkg_configure_make] fix case sensitive comparison in PATH system d…

[flmmkch]

Fixes the icu build error with the following message on my system: Unable to find system dir in the PATH variable! Appending required msys paths!

Hopefully fixes #34450

[flmmkch]

@microsoft-github-policy-service agree company="Regnology"

[BillyONeal]

1. It seems like this is unlikely to fix the indicated issue because it doesn't look like

   message(WARNING "Unable to find system dir in the PATH variable! Appending required msys paths!")

   got triggered there?
2. This looks like the previous author intentionally did not do this; they clearly knew TOUPPER existed and yet chose to form the explicit list of different cases anyway. Did something change (as in, a previous PR that added the toupper stuff that was just incomplete) or similar?

[BillyONeal]

RE: 2: e65af7b / #32750 (comment) This change seems consistent with what @Neumann-A indicated there so it seems OK to me.

[BillyONeal]

This change seems reasonable to me but I would really like an understanding of why this is expected to affect the linked issue given that the warning printed when the correct index is not found was not printed.

[Neumann-A]

This looks like the previous author intentionally did not do this; they clearly knew TOUPPER existed and yet chose to form the explicit list of different cases anyway. Did something change (as in, a previous PR that added the toupper stuff that was just incomplete) or similar?

No I just missed that some kind of normalization to either tolower or toupper should be done to avoid dealing with a lot of mixed cased cases. TOUPPER was just used because upper cased paths have been observed before (e.g. SYSTEM)