[RFC] Use Cotire and automatically generate precompiled headers.

[Jookia]

This fixes #20 in a simple and elegant way, by including Cotire in all the CMake targets. In the future if we want to use single precompiled headers across multiple targets that's possible too, or precompile the Gamekeeper includes, but let's cut to some benchmarks:

Without Cotire:

% mkdir build && cd build && git checkout master && cmake .. -DCMAKE_INSTALL_PREFIX=$PWD/../install 
% time make
make  106.46s user 6.04s system 98% cpu 1:54.28 total
% for i in ../src/**/*.cpp; touch $i # Pretend we've edited all source files.
% time make
make  62.11s user 3.45s system 98% cpu 1:06.56 total
% touch ../src/core/curlfiledownloader.cpp # Pretend to edit a file that includes a lot of Boost.
% time make
make  7.57s user 0.56s system 97% cpu 8.301 total

With Cotire:

% mkdir build && cd build && git checkout pch && cmake .. -DCMAKE_INSTALL_PREFIX=$PWD/../install 
% time make
make  52.43s user 3.98s system 98% cpu 57.363 total
% for i in ../src/**/*.cpp; touch $i # Pretend we've edited all source files.
% time make
make  31.24s user 2.52s system 98% cpu 34.244 total
% touch ../src/core/curlfiledownloader.cpp # Pretend to edit a file that includes a lot of Boost.
% time make
make  2.47s user 0.27s system 98% cpu 2.797 total

With these benchmarks, Cotire effectively halves compile times across the board.

Here's the precompiled header used in gamekeeperCore:

/* cotire.cmake 1.6.3 generated file */
/* libgamelib/build/src/core/cotire/gamekeeperCore_CXX_prefix.cxx */
#pragma GCC system_header
#ifdef __cplusplus
#include "/usr/include/c++/4.9.0/cstdint"
#include "/usr/include/c++/4.9.0/map"
#include "/usr/include/boost/any.hpp"
#include "/usr/include/boost/program_options/variables_map.hpp"
#include "/usr/include/boost/filesystem/path.hpp"
#include "/usr/include/c++/4.9.0/unordered_map"
#include "/usr/include/c++/4.9.0/thread"
#include "/usr/include/boost/algorithm/string/classification.hpp"
#include "/usr/include/boost/algorithm/string/replace.hpp"
#include "/usr/include/boost/algorithm/string/split.hpp"
#include "/usr/include/boost/filesystem/fstream.hpp"
#include "/usr/include/boost/filesystem/operations.hpp"
#include "/usr/include/boost/iostreams/device/array.hpp"
#include "/usr/include/boost/iostreams/stream.hpp"
#include "/usr/include/curl/curl.h"
#include "/usr/include/boost/lexical_cast.hpp"
#include "/usr/include/log4cpp/CategoryStream.hh"
#include "/usr/include/log4cpp/Category.hh"
#include "/usr/include/boost/algorithm/string/erase.hpp"
#include "/usr/include/log4cpp/Configurator.hh"
#include "/usr/include/log4cpp/OstreamAppender.hh"
#include "/usr/include/log4cpp/PatternLayout.hh"
#include "/usr/include/c++/4.9.0/atomic"
#include "/usr/include/c++/4.9.0/mutex"
#include "/usr/include/c++/4.9.0/forward_list"
#include "/usr/include/c++/4.9.0/climits"
#include "/usr/include/c++/4.9.0/cstdlib"
#endif

Some issues with this (not pressing):

1. There's no way to disable Cotire. I'm not sure if this is needed considering the advantages it brings and it's entirely in CMake. But the choice would be nice.
2. There's only precompiled headers being generated for gamekeeperCore, gamekeeperClient and gamekeeperBackend. This may be intentional with cost/benefits weighed by Cotire.
3. I'm not sure of the penalty hit we'd get if one of the generated headers would need to be recompiled.
4. It's only halved. I'm not sure if this is the expected result. Based on this and the previous two points, we might want to find a compiler profiler for sticky spots.
5. I grabbed Cotire out of Git, I'm not sure if including it is the right thing considering versioning.

[karolherbst]

1. what does cotire do with unsupported platforms/compiler?
2. is okay, because pch only make sense for targets with more than just one source file
3. the entire target has to be recompiled
4. I didn't expect much more, because there aren't many files yet and half of the source files are compiled in about a second anyway.
5. I would use the newest (stable?) version and only cherry-pick commits, which are helpful.

In general I like the idea, because it is making pch stuff pretty easy for us. I have some notes though:

• include <gamekeeper/core/common.h> isn't needed in source files, because every header includes it first (rule by wiki page: https://github.com/karolherbst/Gamekeeper-Framework/wiki/Cpp-Code-Style#include-order

• we absolutly need cmake switches for single unit compiles. There are some cmake variables, which can be set target wise. single unit compiles are pretty bad for devs, because you have to recompile the entire targets every time. Also pch could cause more harm for devs if gamekeeper stuff gets put there (like the common.h file), but it seems like that cotire only use external deps, which is nice.

[Jookia]

I've fixed the #includes and added a way to disable Cotire. It's on by default. I'm still working on MinGW support (I need to set up my cross compiler), but I still don't quite understand how to identify single unit compiles...?

[karolherbst]

I noticed, that single unit compiles are separated targets

[karolherbst]

I left some comment, but it looks pretty good now.

[Jookia]

I've rebased it, made it use instead of stringutils and removed the pch.h comment in the utils CMakeLists.

[karolherbst]

one a little minor thing, but this looks pretty good now. I think I will merge this tomorrow

[Jookia]

Fixed.