--- a/build/autoconf/icu.m4
+++ b/build/autoconf/icu.m4
@@ -93,19 +93,22 @@ if test -n "$USE_ICU"; then
 fi
 
 AC_SUBST(MOZ_ICU_VERSION)
 AC_SUBST(ENABLE_INTL_API)
 AC_SUBST(USE_ICU)
 AC_SUBST(ICU_DATA_FILE)
 AC_SUBST(MOZ_ICU_DATA_ARCHIVE)
 
-if test -n "$USE_ICU" -a -z "$MOZ_SYSTEM_ICU"; then
-    if test -z "$YASM" -a -z "$GNU_AS" -a "$COMPILE_ENVIRONMENT"; then
-      AC_MSG_ERROR([Building ICU requires either yasm or a GNU assembler. If you do not have either of those available for this platform you must use --without-intl-api])
-    fi
-    dnl We build ICU as a static library.
-    AC_DEFINE(U_STATIC_IMPLEMENTATION)
+if test -n "$USE_ICU"; then
     dnl Source files that use ICU should have control over which parts of the ICU
     dnl namespace they want to use.
     AC_DEFINE(U_USING_ICU_NAMESPACE,0)
+
+    if test -z "$MOZ_SYSTEM_ICU"; then
+        if test -z "$YASM" -a -z "$GNU_AS" -a "$COMPILE_ENVIRONMENT"; then
+            AC_MSG_ERROR([Building ICU requires either yasm or a GNU assembler. If you do not have either of those available for this platform you must use --without-intl-api])
+        fi
+        dnl We build ICU as a static library.
+        AC_DEFINE(U_STATIC_IMPLEMENTATION)
+    fi
 fi
 ])

--- a/config/system-headers
+++ b/config/system-headers
@@ -1312,24 +1312,26 @@ sys/thr.h
 sys/user.h
 kvm.h
 spawn.h
 err.h
 xlocale.h
 #ifdef MOZ_SYSTEM_ICU
 unicode/locid.h
 unicode/numsys.h
+unicode/plurrule.h
 unicode/timezone.h
 unicode/ucal.h
 unicode/uchar.h
 unicode/uclean.h
 unicode/ucol.h
 unicode/udat.h
 unicode/udatpg.h
 unicode/uenum.h
 unicode/unorm.h
 unicode/unum.h
+unicode/upluralrules.h
 unicode/ustring.h
 unicode/utypes.h
 #endif
 libutil.h
 unwind.h
 fenv.h