Don't probe libintl if it wasn't found

It just makes CMake error out rather than continue without NLS
support.

CMakeLists.txt

@@ -151,10 +151,12 @@ if(ENABLE_NLS)
         set(GETTEXT_FOUND TRUE)
       else()
         find_library(LIBINTL_LIBRARY NAMES intl libintl)
-        check_library_exists(${LIBINTL_LIBRARY} "dgettext" "" LIBINTL_HAS_DGETTEXT)
-        if(LIBINTL_HAS_DGETTEXT)
-          set(GETTEXT_LIBRARIES ${LIBINTL_LIBRARY} ${ICONV_LIBRARIES})
-          set(GETTEXT_FOUND TRUE)
+        if(LIBINTL_LIBRARY)
+          check_library_exists(${LIBINTL_LIBRARY} "dgettext" "" LIBINTL_HAS_DGETTEXT)
+          if(LIBINTL_HAS_DGETTEXT)
+            set(GETTEXT_LIBRARIES ${LIBINTL_LIBRARY} ${ICONV_LIBRARIES})
+            set(GETTEXT_FOUND TRUE)
+          endif()
         endif()
       endif()
       set(CMAKE_REQUIRED_LIBRARIES)