include files

The make program has a capability similar to the #include directive of the C preprocessor. If the string include appears as the first seven letters of a line in a makefile and is followed by a blank or a tab, the rest of the line is assumed to be a filename, which the current invocation of make reads. Macros may be used in filenames. The file descriptors are stacked for reading include files so that no more than 16 levels of nested include files are supported.