Make ICU a hard requirement for parrot

Perl 6 is inherently a Unicode-aware language, and parrot without ICU is not.
We seem to have trouble maintaining tests that do not depend on
Unicode features, which tells us that not having Unicode is not acceptable.

Configure.pl

@@ -195,6 +195,10 @@
             if @errors;
         }
 
+        unless ($impls{parrot}{config}{'parrot::has_icu'}) {
+            push @errors, 'Your parrot was built without Support for ICU.';
+        }
+
         $errors{parrot}{'no gen-nqp'} = @errors && !defined $options{'gen-nqp'};
 
         unless (@errors) {

README.md

@@ -52,6 +52,10 @@ To automatically download and build a fresh Parrot and NQP, run:
 
 It is recommended to first install the libicu-dev and libreadline-dev packages.
 
+Rakudo requires parrot to built with ICU support. Please refer to the Parrot
+documentation (for example
+https://github.com/parrot/parrot/blob/master/README.pod) for more details.
+
 ### Configuring Rakudo to run on the JVM
 
 Note that to run Rakudo on JVM, JDK 1.7 must be installed. To automatically