build.py: Do not rebuild the daemon every time.

Running autogen.sh invalidates the build artifacts, causing them to be
rebuilt every time the 'install' action is invoked, which is wasteful.
Only invoke autogen.sh when the configure script is missing.

Change-Id: Ib92b3d14e9f44f465f467ee0948fe3fceb5d3bea

scripts/install.sh

@@ -94,7 +94,7 @@ mkdir -p contrib/native
 )
 
 # Build the daemon itself.
-./autogen.sh
+test -f configure || ./autogen.sh
 
 if [[ "$OSTYPE" != "darwin"* ]]; then
     # Keep the shared libaries on MAC OSX.