unix, windows: add extra fields to uv_stat_t
build: add install target to the makefile

Before this commit, there was no way to install libuv other than
manually copying required files to the path. This commit introduces
an install target which installs the libuv.a, libuv.so and header files
to the install prefix.

    ``
    make
    make install
    ``

The above will use the default prefix `/usr/local`

    ``
    make
    make install PREFIX=/usr
    ``

The above will install to `/usr`
build: switch to autotools

Switch to the build tool everyone loves to hate.  The Makefile has
served us well over the years but it's been acquiring more and more
features that autotools gives us for free, like easy static+shared
library building, sane install targets, and so on.

This commit drops MinGW support.  If there is demand for it, we'll
re-add it.
build: use AM_PROG_AR conditionally

AM_PROG_AR does not exist in automake v0.11 but without it, you can't
build libuv with automake v0.12+ on Darwin.
test: fix fs_fstat test on sunos

The tests are no longer compiled with -D_XOPEN_SOURCE=500 when building
with autotools. Tell the test explicitly that SunOS systems are
expected to have nanosecond fstat(2) granularity.
test: fix fs_chown when running as root

chown(2) to root is expected to fail - unless you're root, of course.
test: fix spawn_setgid_fails and spawn_setuid_fails

Don't try to set a bogus UID or GID and expect to get a meaningful
error. The test expected EPERM but SunOS returns EINVAL because the
id is outside of the range of valid user/group ids.

Try to switch to UID/GID 0. Give up privileges first if we're root,
else the setuid/setgid system call will succeed when it's expected
to fail.
build: use AM_SILENT_RULES conditionally

It's not supported by older automake versions.