auto merge of #1995 : SimonSapin/servo/mako-py34, r=metajack

On a system where `which python` is Python 3.4.x, the build failed with a very unhelpful error message. The root cause is that Mako 0.8.1 was too old.

Makefile.in

@@ -254,7 +254,7 @@ DONE_script = $(B)src/components/script/libscript.dummy
 DEPS_script = $(CRATE_script) $(SRC_script) $(DONE_SUBMODULES) $(DONE_util) $(DONE_style) $(DONE_net) $(DONE_msg) $(DONE_macros)
 
 RFLAGS_style = $(strip $(CFG_RUSTC_FLAGS)) $(addprefix -L $(B)src/,$(DEPS_SUBMODULES)) -L $(B)src/components/util
-MAKO_ZIP = $(S)src/components/style/Mako-0.8.1.zip
+MAKO_ZIP = $(S)src/components/style/Mako-0.9.1.zip
 MAKO_style = $(S)src/components/style/properties.rs
 MAKO_SRC_style = $(MAKO_style).mako
 SRC_style = $(call rwildcard,$(S)src/components/style/,*.rs) $(MAKO_style)
@@ -309,7 +309,9 @@ $(AUTOGEN_SRC_script): $(BINDINGS_SRC)/%Binding.rs: $(bindinggen_dependencies) \
 globalgen_dependencies := $(addprefix $(BINDINGS_SRC)/, GlobalGen.py Bindings.conf Configuration.py CodegenRust.py parser/WebIDL.py) $(CACHE_DIR)/.done
 
 $(MAKO_style): $(MAKO_SRC_style)
-	PYTHONPATH=$(MAKO_ZIP) python -c "from mako.template import Template; print(Template(filename='$<').render())" > $@
+# Use a temporary file to avoid writing an empty (but more recent) file on failure.
+	PYTHONPATH=$(MAKO_ZIP) $(CFG_PYTHON2) -c "from mako.template import Template; print(Template(filename='$<').render())" > $@.tmp
+	mv $@.tmp $@
 
 
 $(CACHE_DIR)/.done:

src/components/style/.gitignore

@@ -1 +1,2 @@
 properties.rs
+properties.rs.tmp