curl_setup: Ensure no more than one IDN lib is enabled

Prior to this change it was possible for libcurl to be built with both
Windows' native IDN lib (normaliz) and libidn2 enabled. It appears that
doesn't offer any benefit --and could cause a bug-- since libcurl's IDN
handling is written to use either one but not both.

Bug: https://github.com/curl/curl/issues/1441#issuecomment-297689856
Reported-by: Gisle Vanem