Makefile: How to create "eqn.cpp" and "eqn.hpp" as they are not rebuilt when removed

[Bjarni Ingi Gislason]

  In a Makefile:

maintainer-clean-generic:
        @echo "This command is intended for maintainers to use"
        @echo "it deletes files that may require special tools to
rebuild."
        -rm -f src/preproc/eqn/eqn.cpp

  and

src/preproc/eqn/eqn.hpp: src/preproc/eqn/eqn.cpp
        @if test ! -f $@; then rm -f src/preproc/eqn/eqn.cpp; else :; fi
        @if test ! -f $@; then $(MAKE) $(AM_MAKEFLAGS) src/preproc/eqn/eqn.cpp; 
else :; fi

-.-.

  Where are these "special tools" to rebuild files documented?

  The creation of "eqn.hpp" has strange conditions!