Make snarfing tools more robust to varied C preprocessor behavior.

* libguile/guile-snarf.in (modern_snarf): Rewrite sed script to cope
  with newlines in the snarfed code segments, or multiple code segments
  on the same line.

* module/scripts/snarf-check-and-output-texi.scm (process-stream):
  Strip all 'hash' tokens from the stream.
  (do-command): Remove special cases that handled 'hash' tokens
  in a few places.
snarf.h: Declare static const function name vars as SCM_UNUSED.

* libguile/snarf.h (SCM_DEFINE_GSUBR, SCM_DEFINE, SCM_PRIMITIVE_GENERIC,
  SCM_DEFINE_PUBLIC, SCM_PROC, SCM_REGISTER_PROC, SCM_GPROC): Declare
  static const function name variables as SCM_UNUSED to avoid spurious
  warnings.
chars.c: Remove duplicate 'const' specifiers.

* libguile/chars.c (scm_r5rs_charnums, scm_r6rs_charnums)
  (scm_r7rs_charnums, scm_C0_control_charnums, scm_alt_charnums):
  Remove duplicate 'const' specifiers.
Improve compliance with C standards regarding signed integer shifts.

* configure.ac: Add -fwrapv when using GCC (or compatible), if
  supported.

* libguile/numbers.h (SCM_I_MAKINUM): Cast to scm_t_bits (unsigned)
  before shifting, to avoid undefined behavior.