Add configure option to disable all timestamps in libMesh

As discussed at idaholab/moose#4278 disabling the use of ```__TIME__``` and ```__DATE__``` can greatly improve the efficacy of _ccache_ (beyond the effects of ```CCACHE_SLOPPINESS=time_macros```!). 
In this PR I add a configure option ```--disable-timestamps```, which replaces the occurrence of all dynamic timestamps (actually those were only found in ```libmesh_common.h``` but which is included _everywhere_) with dummy strings.