configure: check for icu DEFS

In some cases we need to add additional defs to build against icu if icu has certain options configured.

ICU warns about this when building:

*** WARNING: You must set the following flags before code compiled against this ICU will function properly:

-DU_DISABLE_RENAMING=1

We can fetch these flags from the icu pkgconfig and add them if required.

This fixes symbol errors if ICU is built without renaming.