* Don't "externalize url/rev/sha256 to permit easier override".
Just override 'src' itself. Then you can get the source from anywhere,
not just git. I needed to touch this anyway, because I want to use
fetchzip instead of fetchgit for releases (no need to clone repo).
* Latest release has "make install" improvements, simplifying our
install.
(A Scheme subset for Atmel microcontrollers.)
Use of the actual git HEAD because the actual tarball compiles but
segfault at runtime.
Upload the BLINK.ms sample on arduino board (/dev/ttyACM0):
```sh
$ sudo ./result/bin/microscheme -m UNO -d /dev/ttyACM0 -auc ~/repo/perso/microscheme/examples/BLINK.ms
Microscheme 0.8, (C) Ryan Suchocki
>> Treeshaker: After 4 rounds: 84 globals purged! 22 bytes will be
reserved.
>> 18 lines compiled OK
>> Assembling...
>> Uploading...
avrdude: AVR device initialized and ready to accept instructions
Reading | ################################################## | 100%
0.00s
avrdude: Device signature = 0x1e950f
avrdude: reading input file
"/home/tony/repo/perso/microscheme/examples/BLINK.hex"
avrdude: writing flash (2080 bytes):
Writing | ################################################## | 100%
0.35s
avrdude: 2080 bytes of flash written
avrdude: safemode: Fuses OK (E:00, H:00, L:00)
avrdude done. Thank you.
>> Cleaning Up...
>> Finished.
```
