Minor warning fixes

I've made a couple minor changes to allow your code to compile using ``-Wall -Wextra -Werror`` on GCC:

- Added prototype for SCPI_EventClear()
- Removed 'const' from findCommand(), as it places the argument into a non-const field
- Replaced absolute float compare in units.c with epsilon compare
- Cast isdigit()'s argument to int

Also added the *.d header dependency files that GCC can generate to the .gitignore.