Add configure checks for portability assumptions

Add configure checks for portability assumptions

Check a few portability assumptions:

* Integers are two's complement.  Testing that the bitwise complement
  of the representation of -1 is zero is sufficient, because C99
  6.2.6.2 only allows sign and magnitude, one's complement, and two's
  complement as representations.

* Integer values with sign bit one and value bits zero are valid and
  not trap representations.  C99 6.2.6.2 allows such a value to be a
  trap representation.  Testing that the declared integer value bounds
  are asymmetric in magnitude is sufficient.

* Conversion of an unsigned integer value that is not representable in
  the corresponding signed type preserves the bit pattern.  C99
  6.3.1.3 says this is implementation-defined, or raises an
  implementation-defined signal.  Exhaustively checking for the
  desired behavior is prohibitive, so this spot check will have to do.

* Bytes are 8 bits