build: fix --with-intl=system-icu for x-compile

* add toolset stuff to icu-system.gyp
* update docs

Fixes: https://github.com/nodejs/node/issues/3801
PR-URL: https://github.com/nodejs/node/pull/3808
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>

README.md

@@ -257,6 +257,9 @@ Windows:
 $ pkg-config --modversion icu-i18n && ./configure --with-intl=system-icu
 ```
 
+If you are cross compiling, your `pkg-config` must be able to supply a path
+that works for both your host and target environments.
+
 #### Build with a specific ICU:
 
 You can find other ICU releases at

tools/icu/icu-system.gyp

@@ -9,10 +9,12 @@
     {
       'target_name': 'icuuc',
       'type': 'none',
+      'toolsets': [ 'host', 'target' ],
     },
     {
       'target_name': 'icui18n',
       'type': 'none',
+      'toolsets': [ 'host', 'target' ],
     },
   ],
 }