Add instructions for building with Make + Clang.

Author: mbrukman

Makefile

@@ -16,7 +16,7 @@
 # User-servicable parts:
 ################################################################################
 
-# C/C++ compiler -- clang also works
+# C/C++ compiler; to use Clang, build with `make CC=clang CXX=clang++`
 CXX ?= g++
 CC ?= gcc
 

README.md

@@ -9,17 +9,24 @@ Visit our [discussion forum](https://groups.google.com/forum/#!forum/jsonnet).
 
 ## Building Jsonnet
 
-The GCC C++ (g++) compiler is required to build Jsonnet. Clang is also
-supported.
+You can use either GCC or Clang to build Jsonnet. Note that on recent versions
+of macOS, `/usr/bin/gcc` and `/usr/bin/g++` are actually Clang, so there is no
+difference.
 
 ### Makefile
 
-To build jsonnet with g++, run:
+To build Jsonnet with GCC, run:
 
 ```
 make
 ```
 
+To build Jsonnet with Clang, run:
+
+```
+make CC=clang CXX=clang++
+```
+
 To run the output binary, run:
 
 ```