llvm_coverage_build: Use CC and CXX variables by default

tools/llvm_coverage_build

 #!/bin/bash
 # Copyright (c) 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # Build Skia for use with LLVM's coverage tools.
 #
 # $ tools/llvm_coverage_build [any other flags to pass to make...]
 #
-# This script assumes the use of Clang >=3.6.
+# This script assumes the use of Clang 3.6. Earlier versions do not support
+# the flags we use in the build, and 3.7+ hit asserts during compilation.
 #
 set -e
 
-export CC="$(which clang)"
-export CXX="$(which clang++)"
+export CC="${CC:-$(which clang)}"
+export CXX="${CXX:-$(which clang++)}"
 
 if [[ -z "${CC}" ]] || [[ -z "${CXX}" ]]; then
   echo "Couldn't find Clang on this machine!"
   exit 1
 fi
 
 echo "CC=$CC"
 echo "CXX=$CXX"
 $CC --version
 
 export GYP_DEFINES="skia_clang_build=1"
 export BUILDTYPE=Coverage
 make $@