qes_str: refactor single letter member names

str->l/m/s are now len, capactiy, str

Changes to be committed:
      modified:   qes_file.h
      modified:   qes_seq.c
      modified:   qes_seq.h
      modified:   qes_seqfile.c
      modified:   qes_str.c
      modified:   qes_str.h
      modified:   benchmarks.c
      modified:   test_seq.c
      modified:   test_seqfile.c
Use correct NO_OPENMP macro name to test for OMP

We used LIBQES_NO_OPENMP in some places and NO_OPENMP in others
Re-vamp the way symbols are searched for in CMAKE

Now, we use CHECK_SYMBOL_EXISTS for almost all non-portable symbols. We
store the result in x_FOUND consistently, whereas we used to interchange
this with HAVE_x and NO_x.