Bugfix OpenMP with Visual Studio

[OgreTransporter]

As pointed out in bug #3285, tesseract cannot be built using OpenMP in conjunction with Visual Studio. Exception is the new switch /openmp:llvm of the last Visual Studio update.

This PR disables OPENMP_BUILD for older Visual Studio versions - if enabled - and replaces the switch /openmp with the new switch /openmp:llvm for Visual Studio 2019.

[nocun]

This PR seemingly breaks my local CMake build (Linux):

CMake Error at CMakeLists.txt:259 (if):
  if given arguments:

    "OPENMP_BUILD" "AND" "MSVC" "AND" "LESS" "1929"

  Unknown arguments specified

All server builds seem to be passing though, do they not use CMake or is it my local issue?

Edit: ok I can see the CMake jobs run daily.

[egorpugin]

try "${MSVC_VERSION}"

[nocun]

try "${MSVC_VERSION}"

I think this fixes it, added PR: #3478