Fix the test suite on Darwin

Setting CC makes sure that we never have to perform the Darwin-specific
check for whether we should default to `CC=clang` and thus break `uname`
stubs in unrelated tests.

CI never caught this because Travis already has CC set to gcc-4.6.

test/build.bats

@@ -4,6 +4,7 @@ load test_helper
 export RUBY_BUILD_CACHE_PATH="$TMP/cache"
 export MAKE=make
 export MAKE_OPTS="-j 2"
+export CC=cc
 
 setup() {
   mkdir -p "$INSTALL_ROOT"