package/jsoncpp: switch build system to meson

package/jsoncpp: switch build system to meson

Bumping jsoncpp to 1.9.1 will cause a CMake-related build error:
https://github.com/open-source-parsers/jsoncpp/issues/970

To fix the bug upstream suggests to switch to meson:
"Our official policy has been only supporting the meson build, and
 having users submit fixes for the CMake build."
https://github.com/open-source-parsers/jsoncpp/issues/970#issuecomment-509794015

Remove all _CONF_OPTS as they are not supported by meson.

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>