Make sure the build system honors CPATH and LIBRARY_PATH when
looking for headers and libraries.

--- a/setup.py	2015-10-07 23:32:58.891329173 +0200
+++ b/setup.py	2015-10-07 23:46:29.653349924 +0200
@@ -575,8 +575,8 @@
         # if a file is found in one of those directories, it can
         # be assumed that no additional -I,-L directives are needed.
         if not cross_compiling:
-            lib_dirs = self.compiler.library_dirs + system_lib_dirs
-            inc_dirs = self.compiler.include_dirs + system_include_dirs
+            lib_dirs = os.getenv('LIBRARY_PATH', '').split(os.pathsep)
+            inc_dirs = os.getenv('CPATH', '').split(os.pathsep)
         else:
             # Add the sysroot paths. 'sysroot' is a compiler option used to
             # set the logical path of the standard system headers and