rate limit calls to posix_fadvise()

Use gflags ALIAS instead of ${gflags_XXX} vars

Port stack tracing to Windows

This method ensure that all users of glog get automatically linked to
the DbgHelp library without needing to set compiler flags.

Necessary when building with BUILD_SHARED_LIBS=1.

The gflags project updated their CMake config last year with a
`gflags` ALIAS target. This can be used instead of the legacy
`${gflags_LIBRARIES}` and `${gflags_INCLUDE_DIRS}` variables. It also
looks cleaner.

Fixes #198

Remove files generated by autotools

closes #165 

for #165. I think not many projects have them in their repo.

Backport internal changes

Don't rely on an internal-linkage extern "C" function having an
unmangled name. This isn't required by the ABI, and in fact is not
valid for a conforming compiler(!). Instead, allow symbolization to
produce either a mangled or an unmangled name here.

CHECK_NOTNULL works with smart pointers when compiled in C++11.

Add __declspec(noreturn) on Win

Currently cl.exe doesn't know LOG(FATAL) exits the program. Set
__declspec(noreturn).

DCHECK_ALWAYS_ON to make D* enabled under NDEBUG

The macro NDEBUG could be automatically defined for release build on
some build environments (e.g. MSVC).  If we use NDEBUG as a key to
distinguish using DCHECK as CHECK (I call this DCHECK is enabled) or
not, we cannot make DCHECK enabled for release build on such
environments.

Considering people use a program with glog for presubmit testing or
dogfooding, they should need to do release build with DCHECK enabled.

There can be a large kernel overhead involved in POSIX_FADV_DONTNEED.
There is no point in calling this per item logged, so rate limit
to at most once per 2MiB written.

With a simple test program that logs 100K items at WARNING level:

Before:

  $ time strace -c -e fadvise64 log.test \
    -log_dir=/dev/shm -logtofiles=true -logtostderr=false
  % time     seconds  usecs/call     calls    errors syscall
  ------ ----------- ----------- --------- --------- ----------------
  100.00   12.522509         125     99957           fadvise64
  ------ ----------- ----------- --------- --------- ----------------
  real    0m52.671s
  user    0m2.194s
  sys     0m44.022s

After:

  $ time strace -c -e fadvise64 log.test \
    -log_dir=/dev/shm -logtofiles=true -logtostderr=false

  % time     seconds  usecs/call     calls    errors syscall
  ------ ----------- ----------- --------- --------- ----------------
  100.00    0.000759         152         5           fadvise64
  ------ ----------- ----------- --------- --------- ----------------
  real    0m4.206s
  user    0m1.436s
  sys     0m3.153s

Fixes issue #84

cmake: allow to disable building unit tests

This commit introduces a CMake `BUILD_TESTING` boolean option that
allows to disable unit tests. This is especially useful in superbuilds
where typically only the main library is built. By default, this option
is enabled.

try to avoid the error "conflicting declaration 'typedef DWORD pthread_t'" etc. in MinGW

Set sinks_ to NULL after deletion in LogDestination::DeleteLogDestinations

Fix autotools build.

Added $<BUILD_INTERFACE> to gflags include directory