doc: update minimum g++ version to 4.9.4

The 4.8.x releases don't fully support C++11.  Update the prerequisites
for node.js 8 so that we won't have to work around compiler bugs in the
future.

To be decided if we should also update the minimum clang version.
I believe clang 3.4.2 properly supports C++11 but am not 100% sure.

PR-URL: #13466
Ref: #11840
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Nikolai Vavilov <vvnicholas@gmail.com>

BUILDING.md

@@ -65,7 +65,7 @@ Depending on host platform, the selection of toolchains may vary.
 
 #### Unix
 
-* GCC 4.8.5 or newer
+* GCC 4.9.4 or newer
 * Clang 3.4.2 or newer
 
 #### Windows
@@ -80,8 +80,8 @@ Depending on host platform, the selection of toolchains may vary.
 
 Prerequisites:
 
-* `gcc` and `g++` 4.8.5 or newer, or
-* `clang` and `clang++` 3.4.2 or newer
+* `gcc` and `g++` 4.9.4 or newer, or
+* `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