Modern CMake

[sodevel]

I tried to use this library as subproject during a CMake build but this failed because the current CMake files are pre-Modern-CMake age and the library target doesn't carry the usage requirements. I wanted to fix that and ended up rewriting the main CMake files completely. I tried to stay compatible with the current variant, but i had to change some user visible aspects to follow recommended CMake practices. I also fixed some issues with the library that looked like errors to me. I did not update the CMake files of the examples and tests except two small changes to make them work with the rewritten files. I did not touch the CMake file in the hal directory, it references even more elements not present in the repository and is not necessary to build the library.

User visible changes:

1. All cache variables use a common prefix. This reduces conflicts when used as subproject in another CMake build.
2. TLS support can now be enabled with a cache variable.
3. Honor BUILD_SHARED_LIBS and build only one library variant at a time. This seems to be the recommended way and simplifies the CMake files a lot.
4. Remove homegrown cross-compiling. This doesn't work correctly, use toolchain files for cross-compiling.
5. Add file-service to public API. It looks like this should be public, an example uses this API and won't work otherwise.
6. Change the include path in the PkgConfig file to match the path used by the examples. The examples include the files without path, the PkgConfig file should use the same path, or the client application needs to change the paths depending if the library is used as subproject or library.

Internal changes:

1. Use Mbedtls as subproject. It's just better to use its CMake build system than compiling its files on your own.
2. Apply PLATFORM_IS_BIGENDIAN result. While the test was performed, the result was actually never used.
3. Remove marking of source files as CXX for MSVC. I see no reason why this should be done, maybe ancient MSVC versions lacked some C features?
4. Remove using of Linker def file for MSVC. The file isn't present in the repository anyway.
5. Remove manual configuration of Resource Compiler. This should work just fine on its own in current CMake versions.
6. Remove generation of export headers. These aren't used at all.
7. Don't hardcode path segments for install target. Use CMake variables to specify these locations in a safe manner.
8. Don't hardcode path segments in PkgConfig file. Use the CMake variables used by the install target to stay in sync.

I have tested these new CMake files on Linux only, they are working fine in all configurations and with or without Mbedtls. I couldn't test on other platforms. I also did not test CPack.