fix #133: Use awk instead of sed which would require different options on OS X

mod: Update doc with new find_package COMPONENTS or subproject use

Fix build of unit test on Ubuntu/Windows by changing include order

These changes improve the CMakeLists.txt of the gflags project to make CMake variables configurable via command-line option -D or super-project settings. Moreover, gflags related variables are only added to the CMake cache when the gflags project is configured as a standalone project. When it is a subproject (cf. #117), the super-project is responsible for setting the configuration variables.

This change set also changes the names of the (exported) target names to gflags[_nothreads]_(static|shared) and the CMake (package) configuration variable names to start with an uppercase prefix "GFLAGS_".

find_package COMPONENTS

This change allows the user to include the gflags project as subtree/
submodule in their own project. In this case, no CMake variables used
to configure the build and installation of the gflags library should be
added to the CMake cache.

Fix compilation of unittest on Ubuntu

This reverts commit 02f31897.

Use ```_static``` suffix in filename of static library on Windows to distinguish it from the import library.

Add Bazel build support

Define NO_THREADS when building without threads

Previously NOTHREADS was being defined but the code
in mutex.h checks for NO_THREADS.

Resolve some compiler warnings

MSVC 2015 already defines vsnprintf() and va_copy().