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

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.

This commit addresses a few issues:

    1.  No longer leak config.h in a way similar to
            https://github.com/gflags/gflags/issues/233
        The solution of prefixing the path by 'glog_internal' is modified from
            https://github.com/gflags/gflags/issues/234

    2.  No longer expose internal headers.

    3.  Replace PACKAGE_NAME with native.package_name()

    4.  Uers can choose namespaces via the newly added 'namespace' keyword.

    5.  Replace glob with explicitly listing of files.

    6.  Make the genrules more compact using pythonic list construction.

Compute base addresses from program headers while reading /proc/self/maps.

We previously had logic to compute the base address from program
headers as part of symbolization. The problem is that we need a correct
base address earlier in order to adjust a PC into the image's address
space, as these addresses can appear in unsymbolized output.

There was previously an assumption that only the mapping that
was lowest in the address space did not need to be adjusted. This
assumption is not guaranteed (for example, the kernel may choose to
map an ET_DYN lowest) and in fact turned out to be wrong in binaries
linked with lld because the first mapping is read-only.

The solution is to move the program header reading logic into the
code that reads /proc/self/maps.

There is a change in semantics for clients that install a callback
using the InstallSymbolizeOpenObjectFileCallback function. Any such
clients will need to return a correct base address from the callback
by reading program headers using code similar to that in the function
OpenObjectFileContainingPcAndGetStartAddress.

Cache strlen outside of cycles (PVS-Studio)

Shell escape arguments to /bin/mail.

Fix username lookup in case of missing USER environment variable

#253: Use MS preprocessor idiom to disable warning

Zero allocation fix

Fix for missing exports (fixes #227)

Fix LOG_EVERY_N with clang -Wunused-local-typedef

This compile time assert is no longer used anywhere in glog. Remove
it.