dependencies: Add openmp 5.0

Since it's been released and we know what the version it will return is.

mesonbuild/dependencies/misc.py

@@ -234,6 +234,7 @@ class MPIDependency(ExternalDependency):
 class OpenMPDependency(ExternalDependency):
     # Map date of specification release (which is the macro value) to a version.
     VERSIONS = {
+        '201811': '5.0',
         '201511': '4.5',
         '201307': '4.0',
         '201107': '3.1',