Add CMake support:

cmake support in rippled. Currently supports:

 * unity/nounity debug/release
 * running protobuf
 * sanitizer builds
 * optional release build with assert turned on
 * `target` variable to easily set compiler/debug/unity
    (i.e. -Dtarget=gcc.debug.nounity)
 * gcc/clang/visual studio/xcode
 * linux/mac/win
 * gcc 4 ABI, when needed
 * ninja builds
 * check openssl for acceptably recent release
 * static builds

TBD:
 * jemalloc support
 * count

Notes:
 * Use the -G"Visual Studio 14 2015 Win64" generator on Windows. Without
   this a 32-bit project will be created. There is no way to set the
   generator or force a 64-bit build in CMakeLists.txt (setting
   CMAKE_GENERATOR_PLATFORM won't work). The best solution may be to
   wrap cmake with a script.

 * It is not possible to generate a visual studio project on linux or
   mac. The visual studio generator is only available on windows.

 * The visual studio project can be _either_ unity or
   non-unity (selected at generation time).  It does not appear possible
   to disable compilation based on configuration.

 * Language is _much_ worse than python, poor documentation and "quirky"
   language support (for example, generator expressions can only be used
   in limited contexts and seem to work differently based on
   context (set_property can set multiple values, add_compile_options
   can not/or is buggy)

 * Could not call out to `sed` because cmake messed with the regular
   expression before calling the external command. I did not see a way
   around this.

 * Makefile generators want to be single target. It wants a separate
   directory for each target type. I saw some mentions on the web for
   ways around this bug haven't look into it. The visual studio project
   does support debug/release configurations in the same project (but
   not unity/non-unity).

SConstruct

@@ -684,6 +684,7 @@ def config_env(toolchain, variant, env):
             '/wd"4244"',
             '/wd"4267"',
             '/wd"4800"',            # Disable C4800 (int to bool performance)
+            '/wd"4503"',            # Disable C4503 (Decorated name length exceeded)
             ])
         env.Append(CPPDEFINES={
             '_WIN32_WINNT' : '0x6000',
@@ -1094,18 +1095,6 @@ for tu_style in ['classic', 'unity']:
             for s, k in sources:
                 object_builder.add_source_files(*s, **k)
 
-            git_commit_tag = {}
-            if toolchain != 'msvc':
-                git = Beast.Git(env)
-                if git.exists:
-                    id = '%s+%s.%s' % (git.tags, git.user, git.branch)
-                    git_commit_tag = {'CPPDEFINES':
-                                      {'GIT_COMMIT_ID' : '\'"%s"\'' % id }}
-
-            object_builder.add_source_files(
-                'src/ripple/unity/git_id.cpp',
-                **git_commit_tag)
-
             if use_shp(toolchain):
                 cc_flags = {'CCFLAGS': ['--system-header-prefix=rocksdb2']}
             else: