5.1.1 Standard Symbols

All the generic macros that AC_DEFINE a symbol as a result of their test transform their arguments 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*)

will define the symbol `HAVE_STRUCT__EXPENSIVEP' if the check succeeds.