build: Use the crate version when the 'git describe' command failed

In our build-script (build.rs), we won't set the environment variable
'BUILD_VERSION' when the 'git describe' command failed (e.g. when the
current source tree does not contain git information). This patch added
a fall back path where the default value of 'BUILD_VERSION' is based on
the 'cloud-hypervisor' crate version.

Fixes: #1669

Signed-off-by: Bo Chen <chen.bo@intel.com>

Cargo.toml

@@ -24,6 +24,9 @@ vhost_user_net = { path = "vhost_user_net"}
 vmm = { path = "vmm" }
 vmm-sys-util = "0.6.1"
 
+[build-dependencies]
+clap = { version = "2.33.3", features = ["wrap_help"] }
+
 [patch.crates-io]
 vm-memory = { git = "https://github.com/cloud-hypervisor/vm-memory", branch = "ch" }