Add support for building with UCRT on Windows. * configure.ac: Add new configure switch "--with-windows-msvcrt". It can be set to "msvcrt" (default) or "ucrt". * Makefile.in: Make configure result of new switch available in makefile rules. * src/mingw-w64.mk, src/build-gcc.mk: Select CRT when installing system headers and when compiling system runtime. See also: https://octave.discourse.group/t/1821

## System-wide startup file for Octave.
##
## This file should contain any commands that should be executed each
## time Octave starts for every user at this site.
EXEC_PATH (cstrcat (fullfile (OCTAVE_HOME, 'notepad++'), pathsep, EXEC_PATH));
EXEC_PATH (cstrcat (fullfile (OCTAVE_HOME, 'bin'), pathsep, EXEC_PATH));
EDITOR (fullfile (OCTAVE_HOME, 'notepad++', 'notepad++.exe'));