cmake: think in CMake targets

[madebr]

This is the yder equivalent of babelouest/orcania#30
When downloading dependencies, this has a HARD requirement on babelouest/orcania#30.

The reason for the hard requirement is as follows:
The exported yder targets link to orcania targets. At export time, CMake checks whether all targets are being exported.
Because upstream orcania currently does not export any target, this fails.

These errors look as follows:

CMake Error: install(EXPORT "YderExports" ...) includes target "yder_static" which requires target "orcania" that is not in any export set.
CMake Error: install(EXPORT "YderExports" ...) includes target "yder" which requires target "orcania" that is not in any export set.

I fixed the hard requirement by using imported targets instead of ALIAS'es.

After this pr, an install prefix looks as follows:

prefix
├── include
│   ├── yder-cfg.h
│   └── yder.h
├── lib64
│   ├── cmake
│   │   └── Yder
│   │       ├── FindSystemd.cmake
│   │       ├── YderConfig.cmake
│   │       ├── YderConfigVersion.cmake
│   │       ├── YderTargets.cmake
│   │       └── YderTargets-debug.cmake
│   ├── libyder.a
│   ├── libyder.so -> libyder.so.1.4
│   ├── libyder.so.1.4 -> libyder.so.1.4.18
│   ├── libyder.so.1.4.18
│   └── pkgconfig
│       └── libyder.pc
└── share
    └── doc
        └── yder
            ├── examples
            │   ├── log_callback.c
            │   ├── log_combined.c
            │   ├── log_console.c
            │   ├── log_file.c
            │   ├── log_journald.c
            │   ├── log_newline.c
            │   ├── log_syslog.c
            │   └── Makefile
            └── README.md

Also note that FindSystemd.cmake is installed as well.
This is because a static yder needs a systemd library, even when used by a consumer.

After this pr, a project can use yder by doing:

cmake_minimum_required(VERSION 3.0)
project(my_logger C)
find_package(Yder REQUIRED)

add_executable(my_static_logger my_logger.c)
target_link_libraries(my_static_logger PRIVATE Yder::Yder-static)

add_executable(my_logger my_logger.c)
target_link_libraries(my_logger PRIVATE Yder::Yder)

Note how the cmake script does not need to worry about linking to systemd/orcania.

Used by conan-io/conan-center-index#13614

[babelouest]

Hello @madebr ,

Is it possible to resolve the conflict? It seems that I'm missing some code if I do it manually...

I'd have the same request for babelouest/ulfius#242, if you don't mind

[madebr]

FYI, I only had to add -Wconversion to the new compile options.