build: ignore OSError raised by subprocess.check_output()

For example, when git is not installed.

Change-Id: I2a44b9c647e2b2e0344e2c831217db1595a9db68
diff --git a/wscript b/wscript
index f37418d..ee3f835 100644
--- a/wscript
+++ b/wscript
@@ -343,7 +343,7 @@
             else:
                 # no tags matched
                 Context.g_module.VERSION = '%s-commit-%s' % (VERSION_BASE, out)
-    except subprocess.CalledProcessError:
+    except (OSError, subprocess.CalledProcessError):
         pass
 
     versionFile = ctx.path.find_node('VERSION')