CMake: fix -Wint-conversion warning (error if clang is used as MPIC_C)

Using of single quotes for printf argument within MPI_C check
of CMakeLists.txt results in warning of conversion to int.
The single quotes is used for char/int but not for char* type
where the double quotes should be used.

If clang is used as MPI_C instead of gcc the configuration is failed
with error:
incompatible integer to pointer conversion passing 'int'
to parameter of type 'const char *' [-Wint-conversion]

(refer Gentoo [guru overlay] issue: https://bugs.gentoo.org/888009)

Signed-off-by: Sergey Torokhov <torokhov-s-a@yandex.ru>

Author: band-a-prend

CMakeLists.txt

@@ -422,7 +422,7 @@ int main(int argc, char** argv) {
   char processor_name[MPI_MAX_PROCESSOR_NAME];
   int name_len;
   MPI_Get_processor_name(processor_name, &name_len);
-  printf('Hello world from processor %s, rank %d out of %d processors',
+  printf(\"Hello world from processor %s, rank %d out of %d processors\",
          processor_name, world_rank, world_size);
   MPI_Finalize();
 }"