Use an (over-writable) environment variable for the `gdb` command
Use an (over-writable) environment variable for the `gdb` command

Instead of hard-coding the command to run, using the environment
variable `GDB_CMD` (that defaults to `gdb`) allows using a different
debugger than the default `gdb` executable.

This gives the possibility to use `cgdb` as the debugger, which provides
a nicer user interface. Note that one has to use `GDB_CMD="cgdb --"` to
use cgdb (note the trailing `--`) to let cgdb pass the proper arguments
to `gdb`.

graydon/rust,GBGamer/rust,graydon/rust,aidancully/rust,GBGamer/rust,aidancully/rust,aidancully/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust,graydon/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,graydon/rust,graydon/rust