missing icui18n for gem

[petr.2006]

I am trying to instal gollum in gem:

sudo gem install gollum
Building native extensions.  This could take a while...
ERROR:  Error installing gollum:
	ERROR: Failed to build gem native extension.

    current directory: /Library/Ruby/Gems/2.3.0/gems/charlock_holmes-0.7.6/ext/charlock_holmes
/System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/bin/ruby -r ./siteconf20180420-50387-908xpy.rb extconf.rb
checking for main() in -licui18n... no
checking for main() in -licui18n... no


***************************************************************************************
*********** icu required (brew install icu4c or apt-get install libicu-dev) ***********
***************************************************************************************

I have 

port installed icu
The following ports are currently installed:
  icu @58.2_2 (active)

and 

in /opt/local/lib
ls *icui18*
libicui18n.58.2.dylib	libicui18n.58.dylib	libicui18n.a		libicui18n.dylib

where could be the problem?

Petr