Please add 'gnu_dirs' support

[remicollet]

We need to be able to use --libdir option to waf configure.

In wscript, please add

--- wscript.rpm 2014-09-12 08:37:18.266568098 +0200
+++ wscript 2014-09-12 08:37:23.268590579 +0200
@@ -7,6 +7,7 @@
 import sys

 def options(opt):
+   opt.load('gnu_dirs')
    opt.load('compiler_c')
    opt.add_option(
        '--debug',
@@ -17,6 +18,7 @@

 def configure(conf):
    conf.env.VERSION = VERSION
+   conf.load('gnu_dirs')
    conf.load('compiler_c')
    conf.env.append_unique('CFLAGS', ['-std=gnu99', '-Wall', '-Wextra'])
    if conf.options.debug:

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Thanks