setup: avoid procedure related to hg.exe at setup.py --pure

Before this patch, "setup.py --pure" fails on Windows, because
hgbuildscripts.run() tries to copy "hg.exe", which doesn't generated
at "setup.py --pure".

At that time, run_command('build_hgexe') invoked in
hgbuildscripts.run() does nothing and returns successfully. Therefore,
subsequent procedure assuming existence of "hg.exe" fails.

This patch avoids procedure related to "hg.exe" (= all of
hgbuildscripts.run() except for build_scripts.run() invocation) at
"setup.py --pure".

setup.py

@@ -304,7 +304,7 class hgbuildext(build_ext):
 
 class hgbuildscripts(build_scripts):
     def run(self):
-        if os.name != 'nt':
+        if os.name != 'nt' or self.distribution.pure:
             return build_scripts.run(self)
 
         exebuilt = False