Modern cmake

[Erroneous1]

• Keep off CXX_FLAGS (use target_compile_options instead)
• Don't set flags for debug/release options (let cmake do this)
• Set version in project command and use PROJECT_VERSION* variables
• Install the include directory instead of certain files (not strictly a guideline, but makes sense to me)
• Add alias Fastcgipp::fastcgipp which can be used if included as a subproject
• Use Fastcgipp::fastcgipp when linking examples/tests (lets you link against built package even if fastcgipp is already installed on your system)
• Install to standard GNU directories, letting CMake determine when it needs to use lib, lib64, lib/arch
• Export targets so that another cmake project can find_package(Fastcgipp) and link with Fastcgipp::fastcgipp

I took a few liberties that may break people's build systems (for example, using GNUInstallDirs variables instead of lib${LIB_SUFFIX}), so feel free to have me change things back. I also require CMake 3.8 so that I can use cxx_std_14, but we can always review the various cxx_* options actually needed by the code and add those instead if we need to support older CMake.

[eddic]

Alrighty. Sorry for taking so long on this one @Erroneous1. I wanted to properly understand what you'd done here and have been very busy finishing up an MSc but it's all done now. Anyhoo, good stuff again. No real objections on my part. Thanks!