BUILD: Honour environment variable SWIPL for pip install .

README.md

@@ -53,9 +53,11 @@ using
 
     pip install janus-swi
 
-SWI-Prolog is  selected by  finding `swipl`  on the  executable search
-path.   If `swipl.exe`  is not  in ``%PATH%``  on Windows  the Windows
-registry is examined to find SWI-Prolog.
+SWI-Prolog is  selected from  the environment  variable `SWIPL`  or by
+finding `swipl` on the executable search path.  On Windows, if neither
+of  the above  work,  the Windows  registry is  examined  to find  the
+installed     SWI-Prolog     system    (by     default     `C:\Program
+Files\swipl\bin\swipl.exe`).
 
 If  you installed  SWI-Prolog from  source, it  is advised  to install
 Janus from the  `packages/swipy` directory in the  Prolog source.  The

janus/_find_swipl.py

@@ -3,13 +3,13 @@
 import subprocess
 import re
 
-def swipl_properties():
+def swipl_properties(exe="swipl"):
     try:
-        return swipl_exe_properties()
+        return swipl_exe_properties(exe)
     except:
         exe = find_swipl()
         if ( exe ):
-            return swipl_exe_properties(find_swipl())
+            return swipl_exe_properties(exe)
         return None
 
 def swipl_exe_properties(exe="swipl"):

setup.py

@@ -1,14 +1,16 @@
 # Build janus as a Python package
 
-SWIPL="swipl"
+import os
+
+SWIPL=os.getenv("SWIPL") or "swipl"
 PLLIB="swipl"
 
 from setuptools import setup, Extension
 import sys
 sys.path.append("janus")
 from _find_swipl import swipl_properties
 
-props=swipl_properties()
+props=swipl_properties(SWIPL)
 
 if ( not props ):
     raise RuntimeError("Failed to find SWI-Prolog components")