Update instructions to include how to satisfy gFlags prerequisite on a mac

doc/command_line_tool.md

@@ -41,12 +41,16 @@ repository, you need to run the following command to update submodules:
 git submodule update --init
 ```
 
-You also need to have the gflags library installed on your system. On Linux
-systems, gflags can be installed with the following command:
-
+You also need to have the gflags library installed on your system. gflags can be
+installed with the following command:
+Linux:
 ```
 sudo apt-get install libgflags-dev
 ```
+Mac systems with Homebrew:
+```
+brew install gflags
+```
 
 Once the prerequisites are satisfied, you can build the command line tool with
 the command: