+Each listed required module may be declared a conditional dependency. This
+is indicated by placing the condition for the dependency on the same line,
+enclosed in brackets, after the name of the required module. The condition
+is a shell expression that is run after the module's @code{configure.ac}
+statements. For example:
+@smallexample
+strtoull [test $ac_cv_func_strtoumax = no]
+@end smallexample
+