pgsql: Replace the naive HYPOT() macro with a standards-conformant

Log Message:
-----------
Replace the naive HYPOT() macro with a standards-conformant hypotenuse
function. This avoids unnecessary overflows and probably gives a more
accurate result as well.

Paul Matthews, reviewed by Andrew Geery

Modified Files:
--------------
pgsql/src/backend/utils/adt:
geo_ops.c (r1.108 -> r1.109)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/geo_ops.c?r1=1.108&r2=1.109)
pgsql/src/include/utils:
geo_decls.h (r1.57 -> r1.58)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/geo_decls.h?r1=1.57&r2=1.58)