- check in cmake if localtime_r exists
- if localtime_r is missing (MSVC, mingw-w64 on linux) redefine it using
  localtime_s

- fix dbhelp.h include
- fix dbghelp check and link

This was previously present in
https://github.com/bazelbuild/continuous-integration, but the Bazel team
asked us to move it to our repo. It will be helpful if we need to change
the build configuration, eg to add new platforms.

Resolve missing prototype warning

Fix redefined warnings from config.h

Make int conversion explicit and fix clang warning

The issue was caused by config.h header being included from both
header files and implementation files.

Proposed solution is to have regular header guard in the generated
config.h. Benefit of this solution is that it's least intrusive.
Downside is that it only solves issue for CMake build system, and
autoconf one is not fixed since header template is automatically
generated by autoheader who does not add header guard.

konsole family of terminfo supports colored output

Glog was missing colored output when running in terminal
who is set to konsole* TERM, even though the terminal
itself supports colored output.

Add extra terminfo to the check function, so now output
from Glog is properly colored.

* added dbghelp symbolizer support for mingw and cygwin
* fixed compiler errors in case <stdint.h> is not available
* cmake: check whether SymFromAddr actually works

CMake Update

- Use of Project version properties instead of custom variables
- fix missmatch between VERSION (build version) and SOVERSION (API version)
src: https://cmake.org/cmake/help/latest/prop_tgt/VERSION.html#prop_tgt:VERSION

CTest module already provides it

Useful when using glog as subproject.
Add an explanation at top like in gflags.git CMakeLists.txt

Fixes #289.

This works around https://github.com/bazelbuild/bazel/issues/3979,
and so closes #282.

Use target gflags instead of old VARIABLES

Add an example using glog from Bazel

Update gflags to latest master

This allows to avoid conflicts between third party gflags find modules floating
around. If a package provide a local version of gflags find module, clients
which locate glog will incorrectly use it. Another problem is a CMake error message
"cmake_policy PUSH without matching POP" in nested find_package calls. In summary,
we need to ensure to use the original gflags package config.

This allows to deploy debug and release builds concurrently.

Fixed undeclared identifier error

Update AUTHORS and CONTRIBUTORS for PR#232

This means that gflags no longer leaks config.h, so I had to fix a bug
in glog.bzl where config.h is generated at the wrong path.

I also switched to the best-practice for depending on git repositories,
ie using http_archive with a mirror.

Build with Bazel.