Add support for USES.subpkg

[pizzamig]

Summary

The idea is to have the dependency related *_DEPENDS define as
USES_*_DEPENDS in Uses Mk.
Then, the framework is going to add the USES_*_DEPENDS in the appropriate
SUBPACKAGE one

this patch is just to propose the approach and it's not ready to use
I've changed only LIB_DEPENDS and RUN_DEPENDS

Additionally, we would need a git hook to ensure that USES are not using
*_DEPENDS but USES_*_DEPENDS only

[mat]

So, if I have:

SUBPACKAGES= gtk3 gtk4
USES= gnome=libxml2
USES.gtk3= gnome=gtk30
USES.gtk4= gnome=gtk40

How does that work ?

[pizzamig]

The idea is that every time a USES is included, it generates a USES_*_DEPENDS
Those prefixed depends are going to be copied to the appropriate *_DEPENDS (without suffix main package, with suffixes for the subpackages)

USES= gnome=libxml2

It will set USES_LIB_DEPENDS= libxml2.so:textproc/libxml2and it will be added to LIB_DEPENDS

USES.gtk3= gnome=gtk30

It will set USES_LIB_DEPENDS= libgtk-3.so:x11-toolkits/gtk30and it will be added to LIB_DEPENDS.gtk3

And so on.

[mat]

In D43587#994067, @pizzamig wrote:

The idea is that every time a USES is included, it generates a USES_*_DEPENDS
Those prefixed depends are going to be copied to the appropriate *_DEPENDS (without suffix main package, with suffixes for the subpackages)

USES= gnome=libxml2

It will set USES_LIB_DEPENDS= libxml2.so:textproc/libxml2and it will be added to LIB_DEPENDS

USES.gtk3= gnome=gtk30

It will set USES_LIB_DEPENDS= libgtk-3.so:x11-toolkits/gtk30and it will be added to LIB_DEPENDS.gtk3

And so on.

A USES can only be parsed once, there is a guard against it.

So, in my example, the only one to actually do something is USES= gnome=libxml2, the other two are ignored.

[mat]

Bear in mind that make has no local variables, every variable is global.