using gnulib instead of some compatibility hacks

[slayoo]

As reported on SF.net (https://sourceforge.net/p/gnudatalanguage/feature-requests/112/):

gnulib (https://www.gnu.org/software/gnulib/manual/gnulib.html) should provide us with a more elegant alternative for such hacks as:

datatypes.hpp:
// isfinite & isinf for Solaris
#if defined(__sun__)
# include <ieeefp.h>
# define isfinite finite
# ifndef isinf
# define isinf(x) (!finite(x) && x==x)
# endif
#endif

See also #17