*printf: Add support for MSVC compiler.

*printf: Add support for MSVC compiler.

* m4/printf.m4 (gl_PRINTF_DIRECTIVE_N): On MSVC, install a handler that
handles the exception caused by the %n directive. When cross-compiling,
guess no on native Windows.
(gl_SNPRINTF_TRUNCATION_C99, gl_SNPRINTF_RETVAL_C99,
gl_SNPRINTF_DIRECTIVE_N, gl_SNPRINTF_SIZE1): When snprintf is missing,
emulate it through vsnprintf.
* m4/vsnprintf-posix.m4 (gl_FUNC_VSNPRINTF_POSIX): Update comment.
* doc/posix-functions/dprintf.texi: Update documentation regarding
MSVC 9.
* doc/posix-functions/fprintf.texi: Likewise.
* doc/posix-functions/printf.texi: Likewise.
* doc/posix-functions/snprintf.texi: Likewise.
* doc/posix-functions/sprintf.texi: Likewise.
* doc/posix-functions/swprintf.texi: Likewise.
* doc/posix-functions/vdprintf.texi: Likewise.
* doc/posix-functions/vfprintf.texi: Likewise.
* doc/posix-functions/vprintf.texi: Likewise.
* doc/posix-functions/vsnprintf.texi: Likewise.
* doc/posix-functions/vsprintf.texi: Likewise.
* doc/glibc-functions/asprintf.texi: Likewise.
* doc/glibc-functions/obstack_printf.texi: Likewise.
* doc/glibc-functions/obstack_vprintf.texi: Likewise.
* doc/glibc-functions/vasprintf.texi: Likewise.