libc gettext is never sufficient on musl

--- weechat-1.9/CMakeLists.txt.old	2017-06-25 03:20:52.000000000 -0500
+++ weechat-1.9/CMakeLists.txt	2017-09-13 02:30:43.577284569 -0500
@@ -162,6 +162,7 @@
   find_package(Gettext)
   if(GETTEXT_FOUND)
     add_definitions(-DENABLE_NLS)
+    list(APPEND EXTRA_LIBS intl)
   endif()
 endif()