Don't overwrite CFLAGS and LDFLAGS

Appending to CFLAGS and LDFLAGS allows easy configuration if libs aren't installed to standard locations.

I use boxen to configure my mac, which means that homebrew installed libs are under /opt/boxen/homebrew/include. Currently the makefile overwrites my environment variables so I can't compile Dirt. This fixes it so I can just set CFLAGS and LDFLAGS and compile Dirt fine