Fix install instructions for MacOS

[pboling]

I actually already had the icu4c library installed, so that wasn't the issue. The issue was it couldn't find it.

Fixes build failure:

checking whether to build with ICU support... yes
checking for icu-uc icu-i18n... no
configure: error: ICU library not found
If you have ICU already installed, see config.log for details on the
failure.  It is possible the compiler isn't looking in the proper directory.
Use --without-icu to disable ICU support.