Add a `color' request and a `.color' register to control usage of

Add a `color' request and a `.color' register to control usage of
colours.

* src/roff/troff/input.cc (disable_color_flag): Replaced with...
(color_flag): This (which is the inverse).
(activate_color): New function.
(main, init_input_requests): Updated.
* src/roff/troff/troff.h, src/roff/troff/node.cc
(troff_output_file::fill_color, troff_output_file::glyph_color):
Updated.

* NEWS, doc/groff.texinfo, man/groff_diff.man, man/groff.man:
Document the changes.

Circumvent bug in autoconf 2.53 regarding top_builddir.

* aclocal.m4 (GROFF_BUILDDIR): s/top_builddir/groff_top_builddir/.
* Makefile.in, src/xditview/Imakefile.in, doc/Makefile.in:
s/@top_builddir@/@groff_top_builddir@/.
* configure: Regenerated (with autoconf 2.53).