5.7 Recursive Use of `make'===========================Recursive use of `make' means using `make' as a command in a makefile.This technique is useful when you want separate makefiles for varioussubsystems that compose a larger system.  For example, suppose you havea subdirectory `subdir' which has its own makefile, and you would likethe containing directory's makefile to run `make' on the subdirectory.You can do it by writing this:     subsystem:             cd subdir && $(MAKE)or, equivalently, this (*note Summary of Options: Options Summary.):     subsystem:             $(MAKE) -C subdir      You can write recursive `make' commands just by copying this example,but there are many things to know about how they work and why, and abouthow the sub-`make' relates to the top-level `make'.  You may also findit useful to declare targets that invoke recursive `make' commands as`.PHONY' (for more discussion on when this is useful, see *note PhonyTargets::).   For your convenience, when GNU `make' starts (after it has processedany `-C' options) it sets the variable `CURDIR' to the pathname of thecurrent working directory.  This value is never touched by `make'again: in particular note that if you include files from otherdirectories the value of `CURDIR' does not change.  The value has thesame precedence it would have if it were set in the makefile (bydefault, an environment variable `CURDIR' will not override thisvalue).  Note that setting this variable has no impact on the operationof `make' (it does not cause `make' to change its working directory,for example).