Better errors if dev or gt compilers do not exist

[mikebentley15]

Right now, it will give very strange errors if these compilers do not exist. Such as the following:

/bin/sh: 1: /home/mbentley/git/FLiT/sandbox/simple/g++: not found
/bin/sh: 1: /home/mbentley/git/FLiT/sandbox/simple/g++: not found
expr: syntax error
make: /home/mbentley/git/FLiT/sandbox/simple/g++: Command not found
expr: syntax error
make: /home/mbentley/git/FLiT/sandbox/simple/g++: Command not found
expr: syntax error
expr: syntax error
/bin/sh: 1: /home/mbentley/git/FLiT/sandbox/simple/g++: not found
/bin/sh: 1: /home/mbentley/git/FLiT/sandbox/simple/g++: not found
expr: syntax error
make: /home/mbentley/git/FLiT/sandbox/simple/g++: Command not found
expr: syntax error
make: /home/mbentley/git/FLiT/sandbox/simple/g++: Command not found
expr: syntax error
expr: syntax error

This is not clear. You can see that it's having trouble with finding that executable, but it could be done better.