* This way a potential array copy will be made as high in the call stack
  as possible.

* This makes the m4 more robust with respect to evaluation.

* This way the test also works with pgfortran 19.9.

* The PGI compiler leaves these lying around when it crashes.

* This way $LIBM becomes available earlier.

* Do not create the source files in build tree and only re-generate sources
  in maintainer mode.

* This makes it easier to work with people who can run autoreconf but can't
  be bothered to --enable-maintainer-mode. Arguably that's not
  "the right thing", but this world is full of compromises.

* These were originally put in place by Sergey Kosukhin for YAXT, this
  commit attempts the same for PPM.

* Previously a C++ compiler would pass configure and fail during make
  later on, with this change configure fails early with a more
  readable error message than the C++ compiler produces later on when
  encountering C99 code.

* Based on YAXT contribution by Sergey Kosukhin <sergey.kosukhin@mpimet.mpg.de>.

* This reduces the size of configure significantly by merging redundant tests.