Clean path when building autoconf and automake.

Autoconf hard codes paths into itself. This change is a first
pass at allowing a clean environment to let automake build. The
ability to 'make install DESTDIR=xxx' autoconf then use it to
build automake needs a clean environment. The purpose is to
allow a prefix that needs root without building and packaging
when root.

source-builder/sb/setbuilder.py

@@ -133,6 +133,7 @@ class buildset:
         # exporting to the environment
         os.environ['SB_TMPPREFIX'] = tmpprefix
         os.environ['SB_TMPBINDIR'] = tmpbindir
+        os.environ['SB_ORIG_PATH'] = os.environ['PATH']
         os.environ['PATH'] = path.host(tmpbindir) + os.pathsep + os.environ['PATH']
         self._output('path: ' + os.environ['PATH'])
         # shell format