cmake: Silence find_package_handle_standard_args warnings

[orbea]

Fixes #9734

This silences cmake warnings with cmake-3.17.1 by renaming the variables accordingly.

CMake Warning (dev) at /usr/share/cmake-3.17/Modules/FindPackageHandleStandardArgs.cmake:272 (message):
  The package name passed to `find_package_handle_standard_args` (JSONCPP)
  does not match the name of the calling package (Json).  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/Modules/FindJson.cmake:14 (find_package_handle_standard_args)
  CMakeLists.txt:208 (find_package)
This warning is for project developers.  Use -Wno-dev to suppress it.

CMake Warning (dev) at /usr/share/cmake-3.17/Modules/FindPackageHandleStandardArgs.cmake:272 (message):
  The package name passed to `find_package_handle_standard_args` (LuaJit)
  does not match the name of the calling package (LuaJIT).  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/Modules/FindLuaJIT.cmake:58 (FIND_PACKAGE_HANDLE_STANDARD_ARGS)
  cmake/Modules/FindLua.cmake:11 (find_package)
  CMakeLists.txt:209 (find_package)
This warning is for project developers.  Use -Wno-dev to suppress it.

CMake Warning (dev) at /usr/share/cmake-3.17/Modules/FindPackageHandleStandardArgs.cmake:272 (message):
  The package name passed to `find_package_handle_standard_args` (GetText)
  does not match the name of the calling package (GettextLib).  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/Modules/FindGettextLib.cmake:58 (find_package_handle_standard_args)
  src/CMakeLists.txt:64 (find_package)
This warning is for project developers.  Use -Wno-dev to suppress it.

CMake Warning (dev) at /usr/share/cmake-3.17/Modules/FindPackageHandleStandardArgs.cmake:272 (message):
  The package name passed to `find_package_handle_standard_args` (VORBIS)
  does not match the name of the calling package (Vorbis).  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/Modules/FindVorbis.cmake:23 (find_package_handle_standard_args)
  src/CMakeLists.txt:86 (find_package)
This warning is for project developers.  Use -Wno-dev to suppress it.

To do

Ready for Review.

How to test

Build minetest with a new enough cmake version. Using -DENABLE_SYSTEM_JSONCPP=yes is required to reproduce the jsoncpp warning.

[sfan5]

Works.