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).

src/ripple/app/main/Application.cpp

@@ -73,7 +73,6 @@
 #include <ripple/rpc/Context.h>
 #include <ripple/rpc/RPCHandler.h>
 #include <ripple/shamap/Family.h>
-#include <ripple/unity/git_id.h>
 #include <ripple/websocket/MakeServer.h>
 #include <ripple/crypto/csprng.h>
 #include <ripple/beast/asio/io_latency_probe.h>
@@ -807,7 +806,7 @@ public:
     void onStart () override
     {
         JLOG(m_journal.info())
-            << "Application starting. Build is " << gitCommitID();
+            << "Application starting. Version is " << BuildInfo::getVersionString();
 
         m_sweepTimer.setExpiration (10);
         m_entropyTimer.setRecurringExpiration (300);