Regarding C++, I can confirm that it is on the roadmap and should be available soon.
I took a look at the tutorial that Adafruit provides for the 7-segment display (https://learn.adafruit.com/adafruit-led-backpack?view=all ). They have a note at the bottom of the page saying that the Adafruit_GFX library isn't really used for the 7-segment display. I looked at the code (https://github.com/adafruit/Adafruit_LED_Backpack/blob/master/Adafruit_LEDBackpack.cpp) and it appears that only Adafruit_8x8matrix, Adafruit_8x16matrix, and Adafruit_BicolorMatrix are derived from Adafruit_GFX. If you either comment out those lines (using #ifdef 0 ... #endif) and remove the #include for Adafruit_GFX it should compile without it.
The Adafruit_7segment class is also simple enough that you can probably rewrite it into C without much grief.