Everything builds smoothly now (on both, Ubuntu 14.04.4 and 16.04 / both x86_64). 
BTW. Just for future reference purposes … for the time being, different versions of “/usr/include/gfal2/gfal_api.h” define “GFAL2_API” or “GFAL2_API_H_” … however, in anticipation of any possible future versions … I “protected” the “io/gfal/src/TGFALFile.cxx” file using a “superset” of possible defines: #if defined(_GFAL2_API_) || defined(GFAL2_API_) || defined(_GFAL2_API) || defined(_GFAL2_API_H_) || defined(GFAL2_API_H_) || defined(_GFAL2_API_H)