[CMake] Overhaul; fix several issues

[Spixmaster]

Update my initial pull request, #193.
With learnings from lexbor/lexbor#337.
No build file generated with export() which is installed.

Also fixed this error, https://stackoverflow.com/questions/25676277/cmake-target-include-directories-prints-an-error-when-i-try-to-add-the-source.

[Spixmaster]

The fix turned into an overhaul.

The reason was the following. There were errors like this and variables were not used and thus unnecessarily declared which contributed to the build system not being organised and making corrections really hard.

General corrections

• Update to CMake version 3.30 to get rid of this warning.
• Properly quote variables according to this.
• There were incosistencies with either else() or else(something). Same is true for endif(). I adjusted.
• Use _CURRENT_ versions of directories, see here.
• Use placeholders in mailio-config.cmake.in.
• Apply linter and formatter.

Specific corrections

• MAILIO_DYN_LINK_TESTS makes no sense without MAILIO_BUILD_TESTS. Fixed.
• CMAKE_THREAD_PREFER_PTHREAD does not exist. I know of the StackOverflow question but the documentation does not contain it (anymore). Fixed.
• OpenSSL::Crypto was not needed. Fixed.
• In set(DOXYGEN_OUTPUT_PATH ${DOCOUTDIR}), DOCOUTDIR does not exist. Does it even make sense when set(DOXYGEN_OUTPUT_DIRECTORY ${PROJECT_BINARY_DIR}/docs/mailio) is executed before? I removed it.
• configure_file() has no IMMEDIATE. Removed.
• Use existing variables instead of defining them for input of configure_file().
• version.hpp.in uses PROJECT_VERSION_... instead of mailio_VERSION_... to make project renamings easier.
• Tests and examples were installed even twice. Once in the CMakeLists.txt and the individual files test/CMakeLists.txt and examples/CMakeLists.txt. Removed.
• Applied check_required_components("@PROJECT_NAME@"), reason.
• cmake_dependent_option() is completely useless here and even gives false
  impression of compatibility checks. The function basically just defines default
  values and toggles visibility in the GUI,
  CMakeDependentOption. It does not prevent from configuring incompatible settings. I tested it.

To consider

I am aware of file doxygen.conf.in. It is not used currently so I did not set the needed variables in CMake. I would advise against that and use and independent Doxygen configuration file.

Tests

Everything is backwards compatible.

I tested to build which works.

I also imported the created target in on of my applications which worked too.

[Spixmaster]

Fixes or closes #155 and #126 (should not be closed yet).

[karastojko]

Thanks for the PR. It works on Gcc/Linux but fails with Msvc/Windows, with the same CMake 3.31.3. It complains that it cannot find the Boost path although nothing has changed compared to the previous setup. Let me dig what is going on.

[Spixmaster]

Thanks for your general positive attitude towards this pull request. I would like to assist in this but I do not use Windows.