Tell runtime/cgo to not thread warnings as errors

Occasionally, you want to debug (with delve) your project, but when you attempt to do so, you are greeted with the following error:

 #  warning _FORTIFY_SOURCE requires compiling with optimization (-O)
    ^~~~~~~
cc1: all warnings being treated as errors

To resolve this, just export CGO_CFLAGS with the value "-O2 -g -Wno-error".