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All the generic macros that AC_DEFINE a symbol as a result of
their test transform their argument values to a standard alphabet.
First, argument is converted to upper case and any asterisks
(`*') are each converted to `P'. Any remaining characters
that are not alphanumeric are converted to underscores.
For instance,
AC_CHECK_TYPES([struct $Expensive*])
defines the symbol `HAVE_STRUCT__EXPENSIVEP' if the check succeeds.