build: avoid -Wshift-negative-value warning

Configured with --enable-gcc-warnings, a gcc-6.x build would fail with this:

  gzip.c:118:32: error: left shift of negative value
   #define OFF_T_MIN (~ (off_t) 0 << (sizeof (off_t) * CHAR_BIT - 1))

* gzip.c [OFF_T_MAX]: Define in terms of TYPE_MAXIMUM, not OFF_T_MIN.
[OFF_T_MIN]: Remove now-unused definition.
Include "intprops.h" for definiton of TYPE_MAXIMUM.
* bootstrap.conf (gnulib_modules): Add intprops.