Honor CC from the environment.

Allows for spiffyness

``` bash
CC=clang make
clang -Wall -Wstrict-prototypes -Wmissing-prototypes -Wmissing-declarations -Wshadow -Wpointer-arith -Wcast-qual -Wsign-compare -Iincludes -g -D_GNU_SOURCE=1 -c src/config.c -o src/config.o
...
```