build tag file indexes of source code definitions

 ctags parses source code and produces a sort of index mapping
 the names of significant entities (e.g. functions, classes,
 variables) to the location where that entity is defined. This
 index is used by editors like vi and emacsen to allow moving to
 the definition of a user-specified entity.
 .
 Universal Ctags is a fork of Exuberant Ctags that adds additional
 features as well as support for more languages.