DEV: resolve distutils extension path lazily.

Avoid multiple target-python queries when not needed.

Author: pavoljuhas

src/extensions/SConscript

@@ -30,23 +30,28 @@ test = env.Alias('test', develop,
         '$python -m pyobjcryst.tests.run')
 AlwaysBuild(test)
 
-# normal install - trick distutils into using this module.
-distmodfile = pyoutput('\n'.join([
-    "from setuptools import Distribution, Extension",
-    "ext = Extension('pyobjcryst._pyobjcryst', [])",
-    "attrs = dict(ext_modules=[ext])",
-    "dist = Distribution(attrs)",
-    "bcmd = dist.get_command_obj('build_ext')",
-    "bcmd.finalize_options()",
-    "print(bcmd.get_ext_fullpath(ext.name))",
-    ]))
+def resolve_distutils_target(target, source, env):
+    tgt = pyoutput('\n'.join([
+        "from setuptools import Distribution, Extension",
+        "ext = Extension('pyobjcryst._pyobjcryst', [])",
+        "attrs = dict(ext_modules=[ext])",
+        "dist = Distribution(attrs)",
+        "bcmd = dist.get_command_obj('build_ext')",
+        "bcmd.finalize_options()",
+        "print(bcmd.get_ext_fullpath(ext.name))",
+        ]))
+    env['distsofile'] = env.File(tgt)
+    return 0
+
 cmd_install = '$python setup.py install'
 if 'prefix' in env:
-    cmd_install += ' --prefix=' + env['prefix']
+    cmd_install += ' --prefix=$prefix'
+
 install = env.Alias('install', module, [
-    Mkdir(os.path.dirname(distmodfile)),
-    Copy(distmodfile, '$SOURCE'),
-    Touch(distmodfile),
+    resolve_distutils_target,
+    Mkdir('$distsofile.dir'),
+    Copy('$distsofile', '$SOURCE'),
+    Touch('$distsofile'),
     cmd_install,
     ])
 AlwaysBuild(install)