What is TOOLCHAIN_NEED_CONFIGSITE_CACHE for?

[Phil Blundell <philb@gnu.org>]

This morning I happened to notice that some/all of my images had a
dependency on ncurses for no obviously good reason.  (Following an
earlier small crusade these are now the only remaining dependencies on
ncurses in my build, so I would quite like to get rid of them.)

It turns out that the dependency in question is due to the code in
toolchain-scripts.bbclass (which is inherited from
populate_sdk_base.bbclass, from image.bbclass) which does:

TOOLCHAIN_NEED_CONFIGSITE_CACHE = "${TCLIBC} ncurses"

[...]

python __anonymous () {
    deps = ""
    for dep in (d.getVar('TOOLCHAIN_NEED_CONFIGSITE_CACHE', True) or "").split():
        deps += " %s:do_populate_sysroot" % dep
    d.appendVarFlag('do_configure', 'depends', deps)
}

The code in question was added in:

commit 9e87f1347788beed181476dc4563085db14a4729
Author: Lianhao Lu <lianhao.lu@intel.com>
Date:   Tue Aug 16 11:15:36 2011 +0800

    toolchain-script.bbclass: Collected cached site config in runtime.
    
    [YOCTO #892]
    Modify the function toolchain_create_sdk_siteconfig to collect the
    cached site config files which are specified by
    TOOLCHAIN_NEED_CONFIGSITE_CACHE in runtime.
    
    Also added task dependency to ensure the cached site config files are
    generated.
    
    Signed-off-by: Lianhao Lu <lianhao.lu@intel.com>

... but I don't entirely understand from that commentary what exactly is
special about ncurses that means it needs to be handled this way.  Can
anybody clarify?

thanks

p.