conf: use custom.rst for global variables

fixes: haskellfoundation#76

Author: doyougnu

conf.py

@@ -59,7 +59,7 @@
             'ghcWiki': ('https://gitlab.haskell.org/ghc/ghc/wikis/%s', '#%s'),
             }
 
-# prolog for colored text
+# prolog for colored text and global variables
 rst_prolog = """
 .. include:: <s5defs.txt>
 

custom.rst

@@ -2,3 +2,5 @@
 
 .. role:: strike
     :class: strike
+
+.. |ghcVersion| replace:: 9.2.x

index.rst

@@ -5,7 +5,7 @@ Haskell Optimization Handbook
 *by Jeffrey M. Young*
 
 
-This book assumes you are using ghc ``9.2.x`` or above and using the Linux
+This book assumes you are using ghc |ghcVersion| or above and using the Linux
 operating system. If you are new then the best place to start is the :doc:`How
 to use this book </src/Preliminaries/how_to_use>` chapter. If you are returning
 then feel free to jump to the :ref:`mastertoc` and best wishes. It is dangerous

src/Preliminaries/how_to_use.rst

@@ -30,7 +30,7 @@ optimization rabbit hole. If you have no code to optimize, then you may consider
 picking your favorite Haskell library and attempting to optimize that!
 
 
-The book assumes you are using ``GHC 8.10.x`` and a Linux distribution (kernel
+The book assumes you are using GHC |ghcVersion| and a Linux distribution (kernel
 version ``5.8`` and higher). Should you be using an older compiler than some
 sections, such as :doc:`Using EventLog
 </src/Measurement_Observation/Heap_Ghc/eventlog>`; which arrived in ``GHC 8.8``