Fix compilation to avoid to call non-API entry points

R-Devel CRAN checks return:

checking compiled code ... WARNING
  Fichier 'baseflow/libs/x64/baseflow.dll' :
    Found non-API calls to R: 'FRAME', 'HASHTAB', 'PRCODE', 'PRENV',
      'PRSEEN', 'PRVALUE', 'SYMVALUE'
  These entry points may be removed soon:
  'PRSEEN', 'SYMVALUE'
  
  Compiled code should not call non-API entry points in R.
  
  See 'Writing portable packages' in the 'Writing R Extensions' manual.
  See section 'Moving into C API compliance' in the 'Writing R
  Extensions' manual for issues with use of non-API entry points.