* Fixed some VS2017 compilation errors and warnings.

Signed-off-by: raspopov <raspopov@cherubicsoft.com>

Otherwise, if the package is not installed, configuration will work, but building will fail.

Signed-off-by: Pierre Hallot <hallotpierre@gmail.com>

Some client/app code also optionally use cJSON, however it won't be found if plugins are disabled.

Signed-off-by: Pierre Hallot <hallotpierre@gmail.com>

CMake Warning (dev) at /usr/share/cmake-3.19/Modules/FindPackageHandleStandardArgs.cmake:424 (message):
  The package name passed to `find_package_handle_standard_args` (CJSON) does
  not match the name of the calling package (cJSON).  This can lead to
  problems in calling code that expects `find_package` result variables
  (e.g., `_FOUND`) to follow a certain pattern.
Call Stack (most recent call first):
  cmake/FindcJSON.cmake:23 (FIND_PACKAGE_HANDLE_STANDARD_ARGS)
  CMakeLists.txt:125 (FIND_PACKAGE)
This warning is for project developers.  Use -Wno-dev to suppress it.

Signed-off-by: Pierre Hallot <hallotpierre@gmail.com>

Closes #1911. Thanks to twegener-embertec.

This is for when `notifications_local_only` was set true.

Closes #1902. Thanks to marcinkowskip.

This means libwebsockets < 2.4.0.

Closes #1814. Thanks to Odysseus1710.

- Add `MOSQ_OPT_TLS_USE_OS_CERTS` option, to instruct the client to load and  trust OS provided CA certificates for use with TLS connections.
- All clients now load OS provided CA certificates if used with `-L
  mqtts://...`, or if port is set to 8883 and no other CA certificates are
  used. Closes #1824.
- Add the `--tls-use-os-certs` option to all clients.

Closes #1824. Thanks to Jens Reimann.

Closes #1825. Thanks to Valentin Dimov.