doc: rename namespace from google (deprecated) to gflags (default)

Append LIB_SUFFIX to LIBRARY_INSTALL_DIR.

Fix compatibility of CMakeCXXInformation.cmake module with older versions of CMake

The "SONAME_FLAG" was introduced in CMake 2.8.9, while the current CMake scripts require 2.8.4 (first line of CMakeLists.txt). Increasing the required CMake version will block building in less recent OSes (e.g., for Ubuntu 12.04 LTS, the latest version from the official repositories is currently 2.8.7). The more compatible solution will be to use the old variable name, i.e. the one which was used before CMake was patched.

NB: this issue is triggered upon specifying -DBUILD_SHARED_LIBS=ON.

Related bug report: http://public.kitware.com/Bug/view.php?id=13884 
Commit which implemented this new variable: http://cmake.org/gitweb?p=cmake.git;a=commitdiff;h=e1409ac5

Update issue 79
Added a test which uses gflags_declare.h as any other project would use it to avoid such avoidable build configuration mistakes.

Both GFLAGS_NAMESPACE and GFLAGS_INCLUDE_DIR can be used to customize the installation of the gflags library. One specifies the C++ namespace name of the gflags symbols, the other the include subdirectory path of the public header files. By default, both are set to "gflags", but other Google libraries still expect "google" as C++ namespace name, but not necessarily also include path.

Both GFLAGS_NAMESPACE and GFLAGS_INCLUDE_DIR can be used to customize the installation of the gflags library. One specifies the C++ namespace name of the gflags symbols, the other the include subdirectory path of the public header files. By default, both are set to "gflags", but other Google libraries still expect "google" as C++ namespace name, but not necessarily also include path.

Conflicts:
	gflags-vs2003.sln
	gflags-vs2010.sln
	gflags-vs2012.sln
	vsprojects/gflags_unittest/gflags_unittest-vs2010.vcxproj
	vsprojects/libgflags/libgflags-vs2010.vcxproj

Change version to 2.1.0, i.e., use common semantic versioning system including patch number for bug fix releases.