Missing dependencies on driver header

[<colin.helliwell@ln-systems.com>]

I have a niggly problem I keep running into, especially with fresh image
builds.
I have a custom driver, and a user-space library for controlling it;
therefore the library includes one of the driver's header files (IOCTL defs
etc). 
However I can't get the dependencies correct in the library recipe so as to
get the driver's header installed and available to the library build step.
In other words 'bitbake MyDriverLib' gives a "MyDriver.h: no such file or
directory "
The lib's recipe includes
    inherit autotools lib_package binconfig-disabled pkgconfig
    DEPENDS_${PN} = "MyDriver"
    RDEPENDS_${PN} = "MyDriver"

At 'some point later', the lib *will* build ok:  I've done each MyDriver
step one by one, and it seems that once the 'install' has been done then
MyDriverLib will then compile. ('-c compile MyDriver' isn't sufficient). But
shouldn't the RDEPENDS above trigger the MyDriver install?
I'm obviously missing a dependency hook somewhere?