m4: Search Path

1 
1 9.2 Searching for include files
1 ===============================
1 
1 GNU 'm4' allows included files to be found in other directories than the
1 current working directory.
1 
1    If the '--prepend-include' or '-B' command-line option was provided
1 (⇒Invoking m4 Preprocessor features.), those directories are
1 searched first, in reverse order that those options were listed on the
1 command line.  Then 'm4' looks in the current working directory.  Next
1 comes the directories specified with the '--include' or '-I' option, in
1 the order found on the command line.  Finally, if the 'M4PATH'
1 environment variable is set, it is expected to contain a colon-separated
1 list of directories, which will be searched in order.
1 
1    If the automatic search for include-files causes trouble, the 'p'
1 debug flag (⇒Debug Levels) can help isolate the problem.
1