configure: explicitly test for omp.h

When compiling a package from source on macOS without the custom OpenMP
runtime installed, R provides libomp.dylib but _not_ omp.h. Since we do
include <omp.h> in the package code, test for it at configuration time.

Author: aitap

configure

@@ -73,6 +73,9 @@ fi
 # Specifically use schedule(dynamic) to distinguish incompatibilities in OpenMP runtime on macOS, #7318
 
 cat <<EOF > test-omp.c
+#ifdef _OPENMP
+#include <omp.h>
+#endif
 void test_openmp(int * result) {
  int sum = 0;
 #ifdef _OPENMP