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.

Refs: nodejs#11840

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
@@ -80,7 +80,7 @@ Depending on host platform, the selection of toolchains may vary.
 
 Prerequisites:
 
-* `gcc` and `g++` 4.8.5 or newer, or
+* `gcc` and `g++` 4.9.4 or newer, or
 * `clang` and `clang++` 3.4.2 or newer
 * Python 2.6 or 2.7
 * GNU Make 3.81 or newer