win32/Makefile-newvs.am: Fix 'make dist'

We need to account for the lack of Directory.Build.props in win32/vs10...

win32/Makefile-newvs.am

@@ -41,8 +41,10 @@ endif
 %.props: $(top_builddir)/win32/vs10/Makefile
 	if test -e $(top_srcdir)/win32/vs10/$@; then \
 		sed 's/<VSVer>10<\/VSVer>/<VSVer>$(MSVC_VER)<\/VSVer>/g' < $(top_srcdir)/win32/vs10/$@ > $(top_builddir)/win32/vs$(MSVC_VER)/$@; \
-	else \
+	elif test -e $(top_builddir)/win32/vs10/$@; then \
 		sed 's/<VSVer>10<\/VSVer>/<VSVer>$(MSVC_VER)<\/VSVer>/g' < $(top_builddir)/win32/vs10/$@ > $(top_builddir)/win32/vs$(MSVC_VER)/$@; \
+	else \
+		: \
 	fi
 
 %.vcxproj.filters: $(top_builddir)/win32/vs10/Makefile