Fix compilation with PGI

Fix compilation with PGI

- The GMX_TEST_CFLAG with /ARCH doesn't fail because it isn't
  recognized as flag. Thus it is added and then causes trouble later
  because it is treated as filename.
- Extra { } for omp ordered
- Add warning to not use PGI because it fails the regressiontests
  (12.9 and 12.10) or crashes during the compilation with internal
  compiler errors (12.5, 11.9) (I didn't test more than those
  4 versions).
- I think it is worthwile to fix the first two issues even if PGI
  is not recommended. They could also affect other compilers and
  furture PGI version hopefully don't produce incorrect results.

Change-Id: I6ea24498a6c6712a8ad4b604a0b3c685de1ce0be