- introduce `src/crypto.c` as an umbrella source that does nothing else
than include the selected crypto backend source. Moving this job from
the built-tool to the C preprocessor.
- this allows dropping the various techniques to pick the correct crypto
backend sources in autotools, CMake and other build method. Including
the per-backend `Makefile.<crypto-backend>.inc` makefiles.
- copy a trick from curl and instead of maintaining duplicate source
lists for CMake, convert the GNU Makefile kept for autotools
automatically. Do this in `docs`, `examples` and `src`.
Ref: dfabe8bca2/CMakeLists.txt (L1399-L1413)
Also fixes missing `libssh2_setup.h` from `src/CMakeFiles.txt` after
59666e03f0.
- move `Makefile.inc` from root to `src`.
- reformat `src/Makefile.inc` to list each source in separate lines,
re-align the continuation character and sort the lists alphabetically.
- update `docs/HACKING-CRYPTO` accordingly.
- autotools: update the way we add crypto-backends to `LIBS`.
- delete old CSV headers, indent, and merge two lines in
`docs/Makefile.am` and `src/Makefile.am`.
- add `libssh2.pc` to `.gitignore`, while there.
Closes#941
