Conditional AC_CHECK_HEADER

[Florian Weimer]

The x11vnc configure.ac script contains this:

| if test "x$with_v4l" != "xno"; then
|       AC_CHECK_HEADER(linux/videodev.h,
|               [AC_DEFINE(HAVE_LINUX_VIDEODEV_H)],,)
| fi

This is the point where all the default header check are inserted, so
when the condition is false, they are not run.  Making the
AC_CHECK_HEADER invocation unconditional fixes that.

I want to submit this upstream.  Is there are some explanation somewhere
why it's not permitted to invoke AC_CHECK_HEADER under shell conditional
statement?  I couldn't find it in the manual.

Thanks,
Florian