Environment variables not used to locate gcc/g++

[kitlith]

Checklist

• [x] Check that you have the GNU C Compiler installed by typing gcc --version into a terminal.
• [x] If you don't have the linter package installed, either update to the latest version of linter-gcc or install linter directly with apm install linter.
• [x] Ensure that you are using a valid grammar - either C/C++/C++14
• [x] Ensure that you have read all the relevant information in the wiki, particularly the Common Issues page.

Issue (including debugging messages):

In a nutshell, #45 but with a different variable.

It would be useful to have this in the case where you are working on a project with a cross compiler that is not in your path. (E.G. working for the 3ds with devkitARM)

Workaround:

Add the offending compiler to your path.