build: unify source lists

- 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

os400/make-src.sh

@@ -78,7 +78,7 @@ fi
 
 #       Get source list.
 
-cat ../Makefile.inc ../Makefile.os400qc3.inc                            |
+cat Makefile.inc                                                        |
   sed -e ':begin'                                                       \
     -e '/\\$/{'                                                         \
     -e 's/\\$/ /'                                                       \
@@ -98,7 +98,7 @@ cat ../Makefile.inc ../Makefile.os400qc3.inc                            |
 INCLUDES="'`pwd`'"
 
 for SRC in "${TOPDIR}/os400/os400sys.c" "${TOPDIR}/os400/ccsid.c"       \
-           ${CSOURCES} ${CRYPTO_CSOURCES} macros.c
+           ${CSOURCES} macros.c
 do      MODULE=`db2_name "${SRC}"`
         make_module "${MODULE}" "${SRC}"
 done