--- a/accessible/interfaces/ia2/Makefile.in
+++ b/accessible/interfaces/ia2/Makefile.in
@@ -42,22 +42,16 @@ MIDL_ENUMS = \
   AccessibleStates.idl \
   IA2CommonTypes.idl \
   $(NULL)
 
 MIDL_LIBRARIES = \
   IA2Typelib.idl \
   $(NULL)
 
-CSRCS = \
-  dlldata.c \
-  $(MIDL_INTERFACES:%.idl=%_p.c) \
-  $(MIDL_INTERFACES:%.idl=%_i.c) \
-  $(NULL)
-
 MIDL_GENERATED_FILES = \
   dlldata.c \
   $(MIDL_ENUMS:%.idl=%.h) \
   $(MIDL_INTERFACES:%.idl=%_p.c) \
   $(MIDL_INTERFACES:%.idl=%_i.c) \
   $(MIDL_INTERFACES:%.idl=%.h) \
   $(MIDL_LIBRARIES:%.idl=%.tlb) \
   $(NULL)

--- a/accessible/interfaces/ia2/moz.build
+++ b/accessible/interfaces/ia2/moz.build
@@ -79,16 +79,20 @@ GENERATED_FILES += [
     'AccessibleValue.h',
     'AccessibleValue_i.c',
     'AccessibleValue_p.c',
     'dlldata.c',
     'IA2CommonTypes.h',
     'IA2Typelib.tlb',
 ]
 
+SOURCES += [
+    '!%s' % p for p in GENERATED_FILES if p.endswith('.c')
+]
+
 RCINCLUDE = 'IA2Marshal.rc'
 
 # The Windows MIDL code generator creates things like:
 #
 #   #endif !_MIDL_USE_GUIDDEF_
 #
 # which clang-cl complains about.  MSVC doesn't, so turn this warning off.
 if CONFIG['CC_TYPE'] == 'clang-cl':