Remove references to undefined macros.

Remove references to undefined macros.

The intent is to allow compilation with `-Wundef', which in turn should
make it easier to catch erroneous uses of nonexistent macros.

* libguile/__scm.h: Don't assume `BUILDING_LIBGUILE' is defined.

* libguile/conv-uinteger.i.c (SCM_TO_TYPE_PROTO): Remove unneeded CPP
  conditional on `TYPE_MIN == 0'.

* libguile/fports.c: Check for the definition of `HAVE_CHSIZE' and
  `HAVE_FTRUNCATE', not for their value.

* libguile/ports.c: Likewise.

* libguile/numbers.c (guile_ieee_init): Likewise with `HAVE_DINFINITY'
  and `HAVE_DQNAN'.

* test-suite/standalone/test-conversion.c (ieee_init): Likewise.

* libguile/strings.c: Likewise with `SCM_STRING_LENGTH_HISTOGRAM'.

* libguile/strings.h: Likewise.

* libguile/tags.h: Likewise with `HAVE_INTTYPES_H' and `HAVE_STDINT_H'.

* libguile/threads.c: Likewise with `HAVE_PTHREAD_GET_STACKADDR_NP'.

* libguile/vm-engine.c (VM_NAME): Likewise with `VM_CHECK_IP'.

* libguile/gen-scmconfig.c (main): Use "#ifdef HAVE_", not "#if HAVE_".

* libguile/socket.c (scm_setsockopt): Likewise.