Compile error due to unknown flags in gcc 6.3.0

[pdp7]

Commit 0923b85 ignores new compiler warnings in gcc 8.2.0 but has the regression of causing error on gcc 6.3.0 which is used for the current BeagleBoard.org Debian Stretch images.

debian@beaglebone:~/adafruit-beaglebone-io-python$ sudo make install3
python3 setup.py build # --force
running build
running build_py
creating build
creating build/lib.linux-armv7l-3.5
copying Adafruit_I2C.py -> build/lib.linux-armv7l-3.5
creating build/lib.linux-armv7l-3.5/overlays
copying overlays/builder.py -> build/lib.linux-armv7l-3.5/overlays
copying overlays/__init__.py -> build/lib.linux-armv7l-3.5/overlays
creating build/lib.linux-armv7l-3.5/Adafruit_BBIO
copying Adafruit_BBIO/Encoder.py -> build/lib.linux-armv7l-3.5/Adafruit_BBIO
copying Adafruit_BBIO/__init__.py -> build/lib.linux-armv7l-3.5/Adafruit_BBIO
copying Adafruit_BBIO/sysfs.py -> build/lib.linux-armv7l-3.5/Adafruit_BBIO
running build_ext
building 'Adafruit_BBIO.GPIO' extension
creating build/temp.linux-armv7l-3.5
creating build/temp.linux-armv7l-3.5/source
arm-linux-gnueabihf-gcc -pthread -DNDEBUG -g -fwrapv -O2 -Wall -Wstrict-prototypes -g -fdebug-prefix-map=/build/python3.5-QMv3zq/python3.5-3.5.3=. -fstack-protector-strong -Wformat -Werror=format-security -Wdate-time -D_FORTIFY_SOURCE=2 -fPIC -DBBBVERSION41 -Isource/include/ -I/usr/include/python3.5m -c source/py_gpio.c -o build/temp.linux-armv7l-3.5/source/py_gpio.o -Wall -Werror -Wextra -Wno-missing-field-initializers -Wno-strict-aliasing -Wno-cast-function-type -Wno-error=format-truncation= -Wno-error=sizeof-pointer-memaccess
cc1: error: -Werror=format-truncation=: no option -Wformat-truncation=
cc1: error: unrecognized command line option ‘-Wno-cast-function-type’ [-Werror]
cc1: all warnings being treated as errors
error: command 'arm-linux-gnueabihf-gcc' failed with exit status 1
Makefile:32: recipe for target 'build3' failed
make: *** [build3] Error 1

[pdp7]

Changes in commit 0923b85 to ignore new compiler warnings in gcc 8.2.0 are reverted in commit e06bec0

TODO: determine how to avoid compile failure in gcc 8.2.0 (which is in the Debian 10.x "Buster" test image from @RobertCNelson) without causing a regression with the current Debian 9.x "Stretch" images

[pdp7]

Resolved in e06bec0

For gcc 8.2.0 in buster, see #308