Fix argument passing conventions in THbookFile.

When variable-length strings are passed into functions compiled by
gfortran, the lengths of the strings have to be passed at the end of the
argument list as size_t. When using gcc <= 7 or clang (where the
__GNUC__ macro might return unpredictable values), we were putting int on
the C side, which got interpreted as size_t on the fortran side.

Author: hageboeck

hist/hbook/src/THbookFile.cxx

@@ -140,11 +140,7 @@ static Int_t gLastEntry = -1;
 
 // As recommended in
 // https://gcc.gnu.org/onlinedocs/gfortran/Argument-passing-conventions.html
-#if __GNUC__ > 7
-typedef size_t fortran_charlen_t;
-#else
-typedef int fortran_charlen_t;
-#endif
+using fortran_charlen_t = size_t;
 
 #else
 # define hlimit  HLIMIT