[CMake] Support including Git hash and description into the build.

CMake will automatically pick up changes in git's HEAD so that
the necessary code is rebuilt when the build system is invoked.

Two new options `INCLUDE_GIT_HASH` and `INCLUDE_GIT_DESCRIBE` have been
added that enable/disable including the git hash and the output of `git
describe` respectively. By default if the source tree is a git
repository both options are on, otherwise they are false by default.

To support the `Z3GITHASH` macro a different implementation is used from
the old build system. In that build system the define is passed on the
command line. This would not work well for CMake because CMake
conservatively (and correctly) rebuilds *everything* if the flags given
to the compiler change. This would result in the entire project being
rebuilt everytime git's `HEAD` changed.  Instead in this implementation
a CMake specific version of `version.h.in` (named `version.h.cmake.in`)
is added that uses the `#cmakedefine` feature of CMake's
`configure_file()` command to define `Z3GITHASH` if it is available and
not define it otherwise. This way only object files that depend on
`version.h` get re-built rather than the whole project.

It is unfortunate that the build systems now have different `version.h`
file templates. However they are very simple and I don't want to
modify how templates are handled in the python/Makefile build system.

src/util/version.h.cmake.in

@@ -0,0 +1,8 @@
+// automatically generated file.
+#define Z3_MAJOR_VERSION   @Z3_VERSION_MAJOR@
+#define Z3_MINOR_VERSION   @Z3_VERSION_MINOR@
+#define Z3_BUILD_NUMBER    @Z3_VERSION_PATCH@
+#define Z3_REVISION_NUMBER @Z3_VERSION_TWEAK@
+
+#define Z3_FULL_VERSION    @Z3_FULL_VERSION@
+#cmakedefine Z3GITHASH @Z3GITHASH@