Adding Windows Support

[Netzeband]

Unfortunately the developers of this library did not spend any amount on effort to make this library compilable on Windows. Even the build-system, it not portable.

I updated the windows port from https://github.com/bureau14/leveldb/blob/master, which was based on levelDB 1.18 to version 1.20. The following patches are necessary:

1.) The CMakeList file from https://github.com/bureau14/leveldb/blob/master which works with 1.18
2.) Some changes to this file to make it compatible with 1.20
3.) Several files, which are necessary for windows (also taken form https://github.com/bureau14/leveldb/blob/master) combined with some adoptions to make everything compatible to 1.20.
4.) Some adoptions to the tests. Windows don't like to delete a file, which is currently in use by the same process.

You can find the patches in the attached ZIP.
patches.zip

Furthermore this is a appveyor.yml file, which compiles levelDB 1.20 on AppVeyor CI Build system for Windows 64bit, 32bit as Release and Debug build:

image: Visual Studio 2015
version: 1.0.{build}

environment:
  matrix:
    - PLATFORM: x86
      BUILDTYPE: Debug
    - PLATFORM: x64
      BUILDTYPE: Debug	
    - PLATFORM: x86
      BUILDTYPE: Release
    - PLATFORM: x64
      BUILDTYPE: Release	

clone_folder: c:\projects\leveldb
init:
- if "%PLATFORM%"=="x64" call "C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\vcvarsall.bat" amd64
- if "%PLATFORM%"=="x86" call "C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\vcvarsall.bat" x86
build_script:
- cmd: >-
    cd c:\projects\leveldb

    mkdir build

    cd build

    cmake --version

    cmake -G "NMake Makefiles" .. -DCMAKE_INSTALL_PREFIX=../../install/appveyor -DCMAKE_BUILD_TYPE=%BUILDTYPE% -DBOOST_ROOT=C:\Libraries\boost_1_62_0

    nmake install
test_script:
- cmd: nmake test

I have not tested the CMake-build system on linux so far. But I will do it within the next weeks. If I see issues, I will release some additional patches.

By the way: Even on *nix systems CMake is a better choice as build system than pure Makefiles. It solves many dependencies by itself and on top of it will give you cross-compile possibility for free. Yeah I know, it is hard to use something different, when you are used to pure Makefiles. But give it a try.

[Netzeband]

Today I notices, that the current windows build with the above patches will not enable SNAPPY for levelDB. If you are working with databases which use SNAPPY, this can be an issue.

Thus I created 4 more patch files. All patch files are contained in the following ZIP.
patches.zip

The following files are relevant (in addition to the files, described above):
5.) The library files are now installed during installation step.
6.) All Header files are installed during installation step.
7.) Enable SNAPPY support for windows.
8.) Check environment variable SNAPPY_ROOT to set root dir automatically.

For the Snappy support I used as base the file "FindSnappy.cmake" from repository https://github.com/lemire/FastPFor
This files was published under Apache license. I adapted the file to work together with the precompiles windows version of snappy which can be downloaded from here: https://snappy.machinezoo.com/

[cmumford]

Official support for Windows is probably the most requested enhancement. We are currently working on CMake support with continuous integration via AppVeyor. Using issue #466 to track this work.