make copies of global lists before extending

Not doing so can make source or object files appear multiple times in
the list of files to be compiled or linked, e.g. when doing './waf build
install', subsequently leading to linker errors.

gtk2_ardour/wscript

@@ -422,7 +422,8 @@ def build(bld):
 
         # now the shared library containing the GTK GUI for ardour
         obj = bld (features = 'cxx c cxxshlib')
-        obj.source    = gtk2_ardour_sources
+        # operate on copy to avoid adding sources twice
+        obj.source    = list(gtk2_ardour_sources)
         obj.includes  = [ '../libs/fst', '.' ]
         obj.name      = 'libgtk2_ardour'
         obj.target    = 'gtk2_ardour'
@@ -433,7 +434,8 @@ def build(bld):
             obj = bld (features = 'cxx c cxxprogram winres')
         else:
             obj = bld (features = 'cxx c cxxprogram')
-        obj.source    = gtk2_ardour_sources
+        # operate on copy to avoid adding sources twice
+        obj.source    = list(gtk2_ardour_sources)
         obj.target = 'ardour-' + str (bld.env['VERSION'])
         obj.includes = ['.']
         obj.ldflags = ['-no-undefined']

libs/ardour/wscript

@@ -335,7 +335,8 @@ def build(bld):
     # micro increment <=> no interface changes
     LIBARDOUR_LIB_VERSION = "3.0.0"
 
-    sources = libardour_sources
+    # operate on copy to avoid adding sources twice
+    sources = list(libardour_sources)
     if bld.is_tracks_build():
         sources += [ 'engine_state_controller.cc' ]
     

libs/gtkmm2ext/wscript

@@ -84,7 +84,8 @@ def configure(conf):
 
 
 def build(bld):
-    sources = gtkmm2ext_sources
+    # operate on copy to avoid adding sources twice
+    sources = list(gtkmm2ext_sources)
     if bld.is_tracks_build():
         sources += [ 'waves_fastmeter.cc', 'fader.cc' ]
     else: