* grep -> $(GREP)

Some UNIXy systems (FreeBSD for example) don't use GNU grep by default.
Allow the user to specify which grep implementation to use so that
GNU grep can be specified.

* Allow using versioned avr-gcc command

Don't hardcode "avr-gcc", and allow strings such as "avr-gcc8", or
"avr-gcc-7.3.0" to match checks for "avr-gcc".