doc: update required compiler level for AIX

mhdawson · mhdawson · commit f1a60304b8bb · 2018-04-19T10:47:49.000-04:00
Compilers were updated for 10.X as per discussion in: nodejs/build#925
diff --git a/BUILDING.md b/BUILDING.md
@@ -84,6 +84,9 @@ Depending on host platform, the selection of toolchains may vary.
 * GCC 4.9.4 or newer
 * Clang 3.4.2 or newer
 
+#### AIX
+* GCC 6.3 or newer
+
 #### Windows
 
 * Visual Studio 2017 or the Build Tools thereof
