Add git version to make VERSTR

Add git version descriptor to the VERSTR
make variable.
Rename variable to clarify code

Rename the variable 'rx' to 'regxp' to prevent
masking of a member variable of same name.

Show the name of the device if regexp not match
instead of 'Unknown'.  Rtlsdr device shows as
"rtl=0".
Revert "Add git version to make VERSTR"

It is intentional that we display version 0.0 when people build
from git in release mode.
This reverts commit 4bb00b39042c271bfedafdbd23b6a02e846ee6cc.
Rename regxp to regexp.