Merge pull request #4445 from Jeff-Ciesielski/jeffc/define_assignments

Add the ability to pass a value with the -d flag

doc/manual/pragmas.txt

@@ -1011,3 +1011,30 @@ debugging:
 
   # ... complex code here that produces crashes ...
 
+compile time define pragmas
+---------------------------
+
+The pragmas listed here can be used to optionally accept values from
+the -d/--define option at compile time.
+
+The implementation currently provides the following possible options (various
+others may be added later).
+
+===============  ============================================
+pragma           description
+===============  ============================================
+intdefine        Reads in a build-time define as an integer
+strdefine        Reads in a build-time define as a string
+===============  ============================================
+
+.. code-block:: nim
+   const FooBar {.intdefine.}: int = 5
+   echo FooBar
+
+.. code-block:: bash
+   nim c -d:FooBar=42 foobar.c
+
+In the above example, providing the -d flag causes the symbol
+``FooBar`` to be overwritten at compile time, printing out 42. If the
+``-d:FooBar=42`` were to be omitted, the default value of 5 would be
+used.