build: port to non-GNU make

build: port to non-GNU make

This fixes a problem with native Solaris 'make', which does not
grok '-include' lines (a GNU extension to POSIX 'make').
* configure.ac (man/dynamic-deps.mk): Create it, with an old
time stamp, if doing dynamic dependency tracking.
* man/local.mk (DISTCLEANFILES): Put man/dynamic-deps.mk here,
rather than in CLEANFILES.
(man/dynamic-deps.mk): Don't create it read-only, so that we
can easily touch it later.
Include it with '@AMDEP_TRUE@@am__include@', not with '-include',
as '-include' does not work with native Solaris 'make'.