Avoid builtin dgettext when checking libc

The builtin causes errors in the test, and we're also trying to
determine if it is provided by libc or not.

CMakeLists.txt

@@ -147,6 +147,7 @@ if(ENABLE_NLS)
     find_path(GETTEXT_INCLUDE_DIR libintl.h)
     if(GETTEXT_INCLUDE_DIR)
       set(CMAKE_REQUIRED_LIBRARIES ${ICONV_LIBRARIES})
+      set(CMAKE_REQUIRED_FLAGS -fno-builtin-dgettext)
       check_function_exists(dgettext LIBC_HAS_DGETTEXT)
       if(LIBC_HAS_DGETTEXT)
         set(GETTEXT_FOUND TRUE)
@@ -161,6 +162,7 @@ if(ENABLE_NLS)
         endif()
       endif()
       set(CMAKE_REQUIRED_LIBRARIES)
+      set(CMAKE_REQUIRED_FLAGS)
     endif()
   endif()