`Version_string' as C++ object was not visible to linker from C

`Version_string' as C++ object was not visible to linker from C
object files.
Add `const' to `Version_string'.

* src/utils/pfbtops/pfbtops.c (main), src/roff/groff/groff.cc
(main), src/roff/troff/input.cc (main), src/preproc/tbl/main.cc
(main), src/preproc/pic/main.cc (main), src/preproc/eqn/main.cc
(main), src/preproc/grn/main.cc (main), src/preproc/html/pre-html.cc
(scanArguments), src/preproc/refer/refer.cc (main),
src/preproc/soelim/soelim.cc (main), src/devices/grotty/tty.cc
(main), src/devices/grodvi/dvi.cc (main), src/devices/grolj4/lj4.cc
(main), src/devices/grohtml/post-html.cc (main),
src/devices/grolbp/lbp.cc (main), src/utils/tfmtodit/tfmtodit.cc
(main), src/utils/hpftodit/hpftodit.cc (main),
src/utils/lookbib/lookbib.cc (main), src/utils/indxbib/indxbib.cc
(main), src/utils/lkbib/lkbib.cc (main),
src/utils/addftinfo/addftinfo.cc (main): Implement it.

* src/roff/groff/pipeline.c: Add _UWIN.
* src/include/nonposix.h: Ditto.

* src/include/lib.h: Use HAVE_STRERROR.
* src/roff/groff/pipeline.c: Ditto.
* src/preproc/html/pre-html.cc: Remove declaration of `strerror'.
* src/libs/libgroff/Makefile.sub (version.cc): Add `extern "C"'.
* src/utils/pfbtops/pfbtops.c: Add copyright notice.
* win32-diffs: Updated.
* src/utils/hpftodit/hpftodit.cc (read_map): Fix compiler warning.