Use OS rather than compiler specific flag the same way that
__FreeBSD__, __APPLE__, __OpenBSD__, and __linux__ are used.
_WIN32 is defined by GCC (and others) on windows for Win32
and Win64 applications. _WIN32 is set by default for several
other windows based compilers: DMC, MSVC, Intel, Watcom, LCC.
Although the change is for consistency, it allows the Go tools
to be compiled with non-Mingw GCC distributions and non-GCC
compilers that support the GCC extensions.
R=rsc, brainman, vcc
CC=golang-dev
https://golang.org/cl/2168043
use libbio instead of maintaining own buffer
libbio:
always use vlong offsets; die if off_t is too small
R=r
DELTA=163 (23 added, 63 deleted, 77 changed)
OCL=17508
CL=17512
