Add comment why unistring is linked dynamically

cmake/StaticBuild.cmake

@@ -43,6 +43,8 @@ if(BUILD_STATIC)
 
     set(GETTEXT_LIBRARIES "${GETTEXT_LIBRARIES} -Wl,-Bdynamic")
 
+    # FIXME: MSYS2 doesn't include a static version of this library, so
+    #        we'll have to link it dynamically for now
     if(UNISTRING_LIBRARY)
       set(GETTEXT_LIBRARIES "${GETTEXT_LIBRARIES} -lunistring")
     endif()