Improve H20 build docs

docs/src/installation/building.md

@@ -37,8 +37,14 @@ set RESOLVER_NAME=fileResolver
 To run the build, run:
 
 ~~~admonish warning title="Houdini GCC ABI Change"
-Starting with Houdini 20, SideFX is offering gcc 11 builds. Our automatic builds use this version.
-If you still want to use gcc 9, then you'll have to set the below in our main CMakeLists.txt file.
+Starting with Houdini 20, SideFX is offering gcc 11 builds. Our automatic GitHub builds make use of this starting Houdini 20 and upwards.
+To make our CMake script still work with H19.5, we automatically switch to gcc 9, if the Houdini version 19.5 is in the Houdini root folder path.
+
+If you want to still build against gcc 9 with Houdini 20 and upwards, you'll need to set `_GLIBCXX_USE_CXX11_ABI=0` as described below and make sure you have the right Houdini build installed.
+
+If you want to enforce it manually, you'll need to update the line below in our main CMakeLists.txt file. 
+For gcc 9 builds, you'll need to set it to `_GLIBCXX_USE_CXX11_ABI=0`, for gcc 11 to `_GLIBCXX_USE_CXX11_ABI=1`.
+
 See the official [Release Notes](https://www.sidefx.com/docs/houdini/news/20/platforms.html) for more information.
 ```bash
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