configure: test a real OpenMP operation

It turns out it's possible for omp_get_num_threads() to succeed while
failing to load the shared library that does more involved OpenMP
operations.

tools/check-openmp-flags.R

@@ -18,22 +18,23 @@ writeLines("
 #ifdef _OPENMP
  #include <omp.h>
 #endif
-void test_openmp(int * numprocs) {
- *numprocs =
+void test_openmp(int * result) {
+ int sum = 0;
 #ifdef _OPENMP
-  omp_get_max_threads()
-#else
-  0
+ // Have to test an actual OpenMP operation: simpler tests may succeed for a broken configuration, #6642
+ #pragma omp parallel for reduction(+:sum) num_threads(2)
+ for (int i = 1; i <= 2; ++i) sum += i;
 #endif
- ;
+ *result = sum;
 }
 ", "test.c")
 
 # May fail to compile anyway
 stopifnot(tools::Rcmd("SHLIB --preclean test.c") == 0)
 
 dyn.load(paste0("test", .Platform$dynlib.ext))
-success <- .C("test_openmp", ans = integer(1))$ans > 0
-
-cat(sprintf("Result: %s\n", if (success) "yes" else "no"))
-q("no", status = !success)
+ans <- .C("test_openmp", ans = integer(1))$ans
+desired <- 3L
+cat(sprintf("Test result: %d (must be %d; should be 0 for disabled OpenMP)\n", ans, desired))
+stopifnot(`Test failed` = identical(ans, desired))
+cat("Success\n")