squash! add CLT note

Author: bnoordhuis

BUILDING.md

@@ -81,7 +81,7 @@ Depending on host platform, the selection of toolchains may vary.
 Prerequisites:
 
 * `gcc` and `g++` 4.9.4 or newer, or
-* `clang` and `clang++` 3.4.2 or newer
+* `clang` and `clang++` 3.4.2 or newer (macOS: latest Xcode Command Line Tools)
 * Python 2.6 or 2.7
 * GNU Make 3.81 or newer