Issue #2165 - Build ICU as a shared library if we're building Spidermonkey shared.

It seems that if we're going to enable JS_SHARED_LIBRARY, we need ICU to be a
shared library too in order to prevent code duplication in mozjs.dll bloating
it to a whopping 21 MB.
This unfortunately means we have to fold icudata back to icu itself. But since
icudata is still separated from libxul (which is what really matters for #1451),
I think this is a fair compromise.
Building ICU as shared is offered as an option via --enable-shared-icu. But
technically you really need --enable-shared-icu if you're going to use
--enable-shared-js. Unfortunately I can't make configure exit with an error if
MOZ_SHARED_ICU is not defined when JS_SHARED_LIBRARY is enabled, because options
are controlled by different configure scripts (icu is based on bash, while js is
based on python), and I have no idea how to pass MOZ_SHARED_ICU to js's
moz.configure.

Tag #62

build/moz.configure/old.configure

@@ -226,6 +226,7 @@ def old_configure_options(*options):
     '--enable-safe-browsing',
     '--enable-sandbox',
     '--enable-security-sqlstore',
+    '--enable-shared-icu',
     '--enable-shared-media',
     '--enable-signmar',
     '--enable-simulator',