cmake: allow building static + shared libs in a single pass

- `BUILD_SHARED_LIBS=ON` no longer disables building static lib.

  When set, we build the static lib with PIC enabled.

  For shared lib only, set `BUILD_STATIC_LIBS=OFF`. For static lib
  without PIC, leave this option disabled.

- new setting: `BUILD_STATIC_LIBS`. `ON` by default.

  Force-enabled when building examples or tests (we build those in
  static mode always.)

- fix to exclude Windows Resource from the static lib.

- fix to not overwrite static lib with shared implib on Windows
  platforms using identical suffix for them (MSVS). By using
  `libssh2_imp<.ext>` implib filename.

- add support for `STATIC_LIB_SUFFIX` setting to set an optional suffix
  (e.g. `_static`) for the static lib. (experimental, not documented).
  Overrides the above when set.

- fix to set `dllexport` when building shared lib.

- set `TrackFileAccess=false` for MSVS.

  For faster builds, shorter verbose logs.

- tests: new test linking against shared libssh2: `test_warmup_shared`

- tests: simplify 'runner' lib by merging 3 libs into a single one.

- tests: drop hack from `test_keyboard_interactive_auth_info_request`
  build.

  We no longer need to compile `src/misc.c` because we always link
  libssh2 statically.

- tests: limit `FIXTURE_WORKDIR=` to the `runner` target.

TL;DR: Default behavior unchanged: static (no-PIC), no shared.
       Enabling shared unchanged, but now also builds a static (PIC)
       lib by default.

Based-on: b60dca8b64 #547 by berney on github
Fixes: #547
Fixes: #675
Closes: #863

example/CMakeLists.txt

@@ -68,7 +68,7 @@ foreach(example ${EXAMPLES})
   list(APPEND EXAMPLE_TARGETS example-${example})
   # to find generated header
   target_include_directories(example-${example} PRIVATE ${CMAKE_CURRENT_BINARY_DIR})
-  target_link_libraries(example-${example} libssh2 ${LIBRARIES})
+  target_link_libraries(example-${example} libssh2_static ${LIBRARIES})
 endforeach()
 add_target_to_copy_dependencies(
   TARGET copy_example_dependencies