build/win32/pc_base.py: Allow custom options

Some packages might have some parts that are built for certain build
configs, meaning that they could have .pc files of their own, such as
Pango, where PangoFT2 is optionally built.  Allow such an option if
needed.

Also remove some trailing whitespaces.

build/win32/pc_base.py

@@ -26,7 +26,8 @@ class BasePCItems:
         self.top_srcdir = self.srcdir + '\\..\\..'
         self.version = ''
 
-    def setup(self, argv):
+    def setup(self, argv, parser=None):
+        if parser is None:
             parser = argparse.ArgumentParser(description='Setup basic .pc file info')
         parser.add_argument('--prefix', help='prefix of the installed library',
                             required=True)