gtkdochelper.py: Support Windows cmd.exe consoles
gtkdochelper.py: Support Windows cmd.exe consoles

On Windows, prepend the commands to call the gtk-doc scripts using the
currently-used Python executable, since Windows cmd.exe consoles do not
support shebang lines.

QuLogic/meson,pexip/meson,pexip/meson,pexip/meson,pexip/meson,pexip/meson,becm/meson,jpakkane/meson,becm/meson,QuLogic/meson,pexip/meson,jpakkane/meson,pexip/meson,mesonbuild/meson,QuLogic/meson,becm/meson,mesonbuild/meson,jpakkane/meson,mesonbuild/meson,becm/meson,QuLogic/meson,becm/meson,mesonbuild/meson,jpakkane/meson,QuLogic/meson,QuLogic/meson,jpakkane/meson,mesonbuild/meson,jpakkane/meson,becm/meson,mesonbuild/meson,mesonbuild/meson,becm/meson,QuLogic/meson,jpakkane/meson,becm/meson,pexip/meson,mesonbuild/meson,QuLogic/meson,jpakkane/meson,pexip/meson,pexip/meson,jpakkane/meson,mesonbuild/meson,mesonbuild/meson,becm/meson,becm/meson,QuLogic/meson