libcpp: dependency emission tidying

libcpp: dependency emission tidying

This patch cleans up the interface to the dependency generation a
little.  We now only check the option in one place, and the
cpp_get_deps function returns nullptr if there are no dependencies.  I
also reworded the -MT and -MQ help text to be make agnostic -- as
there are ideas about emitting, say, JSON.

	libcpp/
	* include/mkdeps.h: Include cpplib.h
	(deps_write): Adjust first parm type.
	* mkdeps.c: Include internal.h
	(make_write): Adjust first parm type.  Check phony option
	directly.
	(deps_write): Adjust first parm type.
	* init.c (cpp_read_main_file): Use get_deps.
	* directives.c (cpp_get_deps): Check option before initializing.
	gcc/c-family/
	* c.opt (MQ,MT): Reword description to be make-agnostic.
	gcc/fortran/
	* cpp.c (gfc_cpp_add_dep): Only add dependency if we're recording
	them.
	(gfc_cpp_init): Likewise for target.