Mixer/wscript: Check for presence of ladspa.h.

mixer/wscript

@@ -25,6 +25,7 @@ def configure(conf):
     conf.load('compiler_cxx')
     conf.load('gnu_dirs')
 
+    conf.check(header_name='ladspa.h', define_name='HAVE_LADSPA_H', mandatory=True)
     conf.check_cfg(package='lrdf', uselib_store='LRDF',args="--cflags --libs",
                       atleast_version='0.4.0', mandatory=True)