cmake: look for iconv in libc, too (bug #5316.)

CMakeLists.txt

@@ -841,7 +841,10 @@ if(LIBC)
     endif()
 
     check_library_exists(iconv iconv_open "" HAVE_LIBICONV)
-    if(HAVE_LIBICONV)
+    check_library_exists(c iconv_open "" HAVE_BUILTIN_ICONV)
+    if(HAVE_BUILTIN_ICONV)
+      set(HAVE_ICONV 1)
+    elseif(HAVE_LIBICONV)
       list(APPEND EXTRA_LIBS iconv)
       set(HAVE_ICONV 1)
     endif()