Fix FTBFS for build outside the source code.

This resolves issue #3.

Signed-off-by: Michele Calgaro <michele.calgaro@yahoo.it>
(cherry picked from commit 32d72527ff)

siputils.py

@@ -1029,6 +1029,7 @@ class Makefile:
 
         for f in self.optional_list("INCDIR"):
             cppflags.append("-I" + _quote(f))
+        cppflags.append("-I" + self._src_dir)
 
         libs = []