[Rpm-maint] [rpm-software-management/rpm] Build: Add include dirs to exported lib targets (PR #3222)

[Frank Dana]

Each library target's exported `INTERFACE_INCLUDE_DIRECTORIES` list, as written into the `rpm-targets.cmake` export file, is controlled by the `PUBLIC` arguments supplied to `target_include_directories()` for the library target.

* Directories scoped with the `$<BUILD_INTERFACE>` generator expression will be included in the export file written to the build dir by `export(TARGETS...)`
* Directories scoped with `$<INSTALL_INTERFACE>` will be included in the `rpm-targets.cmake` file installed into the `${CMAKE_INSTALL_LIBDIR}/cmake/rpm/` configuration directory.

Providing `PUBLIC` include directory paths for both the build and install contexts is important, because without them the exported CMake configuration is useless to library consumers.

(If the build is installed into `/usr/` or `/usr/local` it won't matter, because the include directories required are the default `/usr/include` or `/usr/local/include/` paths. But an install targeted elsewhere with a `CMAKE_INSTALL_PREFIX` path needs an `INTERFACE_INCLUDE_DIRECTORIES` property of
`${CMAKE_INSTALL_INCLUDEDIR}/include` to correctly locate its installed includes.)


## Example build
```console
$ cmake -B _build -S . -DCMAKE_INSTALL_PREFIX=/tmp/rpm \
    -DENABLE_PYTHON=0 -DWITH_DBUS=0
$ cmake --build _build
$ cmake --install _build
```

### Without this change

#### In `_build/rpm-targets.cmake`
* The `rpm::librpm` target's `INTERFACE_INCLUDE_DIRECTORIES` contains only `/usr/include` (from the `${POPT_INCLUDE_DIRS}` variable).
* The other targets have no `INTERFACE_INCLUDE_DIRECTORIES` at all.

#### In `/tmp/rpm/lib64/cmake/rpm-targets.cmake`
* The `rpm::librpm` target's `INTERFACE_INCLUDE_DIRECTORIES` contains only `/usr/include`.
* The other targets have no `INTERFACE_INCLUDE_DIRECTORIES` at all.


### With this change

#### In `_build/rpm-targets.cmake`
* The `rpm::librpm` target's `INTERFACE_INCLUDE_DIRECTORIES` contains
  both `/usr/include` and (the expansion of) `${CMAKE_SOURCE_DIR}/include`.
* The other targets also all have `INTERFACE_INCLUDE_DIRECTORIES` set to
  the expansion of `${CMAKE_SOURCE_DIR}/include`.

#### In `/tmp/rpm/lib64/cmake/rpm-targets.cmake`
* The `rpm::librpm` target's `INTERFACE_INCLUDE_DIRECTORIES` contains
  both `/usr/include` and `${_IMPORT_PREFIX}/include`, which corresponds
  to `${CMAKE_INSTALL_PREFIX}/include` for the installed package.
* The other targets also all have `INTERFACE_INCLUDE_DIRECTORIES` set to
  `${_IMPORT_PREFIX}/include`.

This means that the CMake `CONFIG`-mode exports now match the `rpm.pc` configuration provided by `pkg-config`. A downstream consumer's CMake build can use `find_package(rpm CONFIG)` to discover the `IMPORTED` `rpm::` targets from either the package build dir, or its installed path (even if directed to a non-standard location), and properly compile against the RPM libraries present in either of those locations.

You can view, comment on, or merge this pull request online at:

  https://github.com/rpm-software-management/rpm/pull/3222

-- Commit Summary --

  * Build: Add include dirs to exported lib targets

-- File Changes --

    M build/CMakeLists.txt (6)
    M lib/CMakeLists.txt (10)
    M rpmio/CMakeLists.txt (6)
    M sign/CMakeLists.txt (6)

-- Patch Links --

https://github.com/rpm-software-management/rpm/pull/3222.patch
https://github.com/rpm-software-management/rpm/pull/3222.diff

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