FindBLAS: Fix detection of OpenMP as dependency of BLA_STATIC

Update the change from commit f7f3d898 (FindBLAS: Add dependency of
OpenBLAS on OpenMP for BLA_STATIC, 2020-11-10, v3.20.0-rc1~492^2):

* If C is not enabled, find CXX OpenMP libraries instead.

* Do not use BLA_STATIC's custom CMAKE_FIND_LIBRARY_SUFFIXES for OpenMP.
  It can break projects that already call `find_package(OpenMP)` and
  expect a shared library.  Whether OpenMP is static is orthogonal to
  whether BLAS is static.

Fixes: #22039
Issue: #16221

Modules/FindBLAS.cmake

@@ -619,8 +619,17 @@ if(BLA_VENDOR STREQUAL "OpenBLAS" OR BLA_VENDOR STREQUAL "All")
     endif()
     set(_threadlibs "${CMAKE_THREAD_LIBS_INIT}")
     if(BLA_STATIC)
-      find_package(OpenMP COMPONENTS C)
-      list(PREPEND _threadlibs "${OpenMP_C_LIBRARIES}")
+      set(_blas_STATIC_CMAKE_FIND_LIBRARY_SUFFIXES "${CMAKE_FIND_LIBRARY_SUFFIXES}")
+      set(CMAKE_FIND_LIBRARY_SUFFIXES "${_blas_ORIG_CMAKE_FIND_LIBRARY_SUFFIXES}")
+      if (CMAKE_C_COMPILER_LOADED)
+        find_package(OpenMP COMPONENTS C)
+        list(PREPEND _threadlibs "${OpenMP_C_LIBRARIES}")
+      elseif(CMAKE_CXX_COMPILER_LOADED)
+        find_package(OpenMP COMPONENTS CXX)
+        list(PREPEND _threadlibs "${OpenMP_CXX_LIBRARIES}")
+      endif()
+      set(CMAKE_FIND_LIBRARY_SUFFIXES "${_blas_STATIC_CMAKE_FIND_LIBRARY_SUFFIXES}")
+      unset(_blas_STATIC_CMAKE_FIND_LIBRARY_SUFFIXES)
     endif()
     check_blas_libraries(
       BLAS_LIBRARIES