Update and slightly clarify instructions in build_detect_platform (#1301)

build_tools/build_detect_platform

@@ -30,11 +30,10 @@
 # Our project depends on gflags, which requires users to take some extra steps
 # before they can compile the whole repository:
 #   1. Install gflags. You may download it from here:
-#      https://code.google.com/p/gflags/
-#   2. Once install, add the include path/lib path for gflags to CPATH and
-#      LIBRARY_PATH respectively. If installed with default mode, the
-#      lib and include path will be /usr/local/lib and /usr/local/include
-# Mac user can do this by having brew installed and running brew install gflags
+#      https://gflags.github.io/gflags/ (Mac users can `brew install gflags`)
+#   2. Once installed, add the include path for gflags to your CPATH env var and
+#      the lib path to LIBRARY_PATH. If installed with default settings, the lib
+#      will be /usr/local/lib and the include path will be /usr/local/include
 
 OUTPUT=$1
 if test -z "$OUTPUT"; then