Fixes part of #475, along with #477
[LinuxCNC/linuxcnc PR#476] configure.in: require `intltool-extract` executable
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![[gnea/grbl-Mega Issue#1] $ command extentions](https://www.grblhal.com/wp-content/themes/gitphp/timthumb.php?src=https://www.grblhal.com/wp-content/themes/gitphp/assets/img/pic/9.jpg&h=110&w=185&q=90&zc=1&ct=1)
[gnea/grbl-Mega Issue#1] $ command extentions -
![[gnea/grbl-Mega Issue#2] Better interrupt priorities](https://www.grblhal.com/wp-content/themes/gitphp/timthumb.php?src=https://www.grblhal.com/wp-content/themes/gitphp/assets/img/pic/5.jpg&h=110&w=185&q=90&zc=1&ct=1)
[gnea/grbl-Mega Issue#2] Better interrupt priorities -
![[gnea/grbl-Mega Issue#3] ATMega16U2 virtual com port on ATMega2560](https://www.grblhal.com/wp-content/themes/gitphp/timthumb.php?src=https://www.grblhal.com/wp-content/themes/gitphp/assets/img/pic/1.jpg&h=110&w=185&q=90&zc=1&ct=1)
[gnea/grbl-Mega Issue#3] ATMega16U2 virtual com port on ATMega2560 -
![[gnea/grbl-Mega Issue#4] Real-time adjustable feedrates](https://www.grblhal.com/wp-content/themes/gitphp/timthumb.php?src=https://www.grblhal.com/wp-content/themes/gitphp/assets/img/pic/3.jpg&h=110&w=185&q=90&zc=1&ct=1)
[gnea/grbl-Mega Issue#4] Real-time adjustable feedrates
- [gnea/grbl-Mega Issue#1] $ command extentions
- [gnea/grbl-Mega Issue#2] Better interrupt priorities
- [gnea/grbl-Mega Issue#3] ATMega16U2 virtual com port on ATMega2560
- [gnea/grbl-Mega Issue#4] Real-time adjustable feedrates
- [gnea/grbl-Mega Issue#5] question about the download zip contents
- [gnea/grbl-Mega Issue#6] Adding outputs for external state signals
- [gnea/grbl-Mega Issue#7] which g-code sender is being used in grbl-Mega testing?
- [gnea/grbl-Mega Issue#8] Frecuencia y velocidad de avance.