ifnames2.13 can help when writing a configure.in for a software package. It prints the identifiers that the package already uses in C preprocessor conditionals. If a package has already been set up to have some portability, this program can help you figure out what its configure needs to check for. It may help fill in some gaps in a configure.in generated by autoscan2.13 (see autoscan2.13(1)).
ifnames2.13 scans all of the C source files named on the command line (or the standard input, if none are given) and writes to the standard output a sorted list of all the identifiers that appear in those files in #if, #elif, #ifdef, or #ifndef directives. It prints each identifier on a line, followed by a space-separated list of the files in which that identifier occurs.
ifnames2.13 accepts the following options:
Print a summary of the command line options and exit.
Look for the installed macro files in directory DIR. You can also set the AC_MACRODIR environment variable to a directory; this option overrides the environment variable.