FindOpenMP: Add iomp5 variant of Clang OpenMP flags

Try `-fopenmp=libiomp5` in addition to `-fopenmp=libomp`.  Leave the
latter as the default that we try first when the compiler id is `Clang`.
If that does not work we will still fall back to trying all the
candidates and eventually get to the iomp5 variant.

Modules/FindOpenMP.cmake

@@ -48,6 +48,7 @@ function(_OPENMP_FLAG_CANDIDATES LANG)
     #GNU
     "-fopenmp"
     #Clang
+    "-fopenmp=libiomp5"
     "-fopenmp=libomp"
     #Microsoft Visual Studio
     "/openmp"