(reapply with fixes) Build system changes to support mingw build target

libs/pbd/wscript

@@ -38,7 +38,6 @@ libpbd_sources = [
     'controllable.cc',
     'controllable_descriptor.cc',
     'clear_dir.cc',
-    'crossthread.cc',
     'cpus.cc',
     'debug.cc',
     'enumwriter.cc',
@@ -122,6 +121,9 @@ def build(bld):
     if bld.is_defined('DEBUG_RT_ALLOC'):
         obj.source += 'debug_rt_alloc.c'
 
+    if bld.env['build_target'] != 'mingw':
+        obj.source += [ 'crossthread.cc' ]
+
     obj.export_includes = ['.']
     obj.includes     = ['.']
     obj.name         = 'libpbd'
@@ -156,7 +158,7 @@ def build(bld):
         testobj.uselib       = 'CPPUNIT XML SNDFILE'
         testobj.use          = 'libpbd'
         testobj.name         = 'libpbd-tests'
-        if sys.platform != 'darwin':
+        if sys.platform != 'darwin' and bld.env['build_target'] != 'mingw':
             testobj.linkflags    = ['-lrt']