When we generate the Visual Studio 2013 projects, we need to remove the
*.vs12.sourcefiles and *.vs12.sourcefile.filters that are generated during
the process, so that 'make distcheck' won't complain about leftover files.
$(CPP) -P - <$(top_srcdir)/build/win32/vs12/$(1).vcxprojin >$(top_builddir)/build/win32/vs12/$(1).vcxproj
$(CPP) -P - <$(top_srcdir)/build/win32/vs12/$(1).vcxproj.filtersin >$(top_builddir)/build/win32/vs12/$(1).vcxproj.filters
+ $(RM) $(1).vs12.sourcefiles
+ $(RM) $(1).vs12.sourcefiles.filters
$(top_builddir)/build/win32/vs12/$(1).vs12.headers: Makefile
-$(RM) $(top_builddir)/build/win32/vs12/$(1).vs12.headers