meson.build: Disable OpenMP on MSVC builds

The implementation of OpenMP is not compliant for our uses, so disable
it for now by just not checking for it on MSVC builds, as we implicitly
add an /openmp switch to the build, which will cause linking the tests
programs to fail, as the OpenMP implementation is not enough.

meson.build

@@ -365,14 +365,21 @@ if get_option('gnuplot')
   config.set('PIXMAN_GNUPLOT', 1)
 endif
 
-dep_openmp = dependency('openmp', required : get_option('openmp'))
-if dep_openmp.found()
-  config.set10('USE_OPENMP', true)
-elif meson.version().version_compare('<0.51.0')
-# In versions of meson before 0.51 the openmp dependency can still
-# inject arguments in the the auto case when it is not found, the
-# detection does work correctly in that case however, so we just
-# replace dep_openmp with null_dep to work around this.
+if cc.get_id() != 'msvc'
+  dep_openmp = dependency('openmp', required : get_option('openmp'))
+  if dep_openmp.found()
+    config.set10('USE_OPENMP', true)
+  elif meson.version().version_compare('<0.51.0')
+  # In versions of meson before 0.51 the openmp dependency can still
+  # inject arguments in the the auto case when it is not found, the
+  # detection does work correctly in that case however, so we just
+  # replace dep_openmp with null_dep to work around this.
+    dep_openmp = null_dep
+  endif
+else
+  # the MSVC implementation of openmp is not compliant enough for our
+  # uses here, so we disable it here.
+  # Please see: https://stackoverflow.com/questions/12560243/using-threadprivate-directive-in-visual-studio
   dep_openmp = null_dep
 endif