doc: gcc version is at least 4.8.5 in BUILDING.md

>= 4.8.5 is required because of compiler bugs in earlier versions

PR-URL: #11840
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

BUILDING.md

@@ -78,7 +78,7 @@ Depending on host platform, the selection of toolchains may vary.
 
 Prerequisites:
 
-* `gcc` and `g++` 4.8 or newer, or
+* `gcc` and `g++` 4.8.5 or newer, or
 * `clang` and `clang++` 3.4 or newer
 * Python 2.6 or 2.7
 * GNU Make 3.81 or newer