Makethird: fix Extract build error if python not 3.6 or later.

Don't attempt to run thirdparty/extract/src/docx_template_build.py if we don't
have python-3.6 or later. The generated files are in git so this works ok.

Makethird

@@ -244,18 +244,27 @@ THIRD_SRC += $(EXTRACT_SRC)
 $(OUT)/thirdparty/extract/%.o: thirdparty/extract/%.c
 	$(CC_CMD) $(WARNING_CFLAGS) $(LIB_CFLAGS) $(EXTRACT_CFLAGS) $(ZLIB_CFLAGS) $(EXTRACT_BUILD_CFLAGS)
 
+# The first python3 command in the next two rules returns 0 (success) iff
+# python3 exists and is version 3.6 or later.
+
 thirdparty/extract/src/docx_template.c: thirdparty/extract/src/template.docx thirdparty/extract/src/docx_template_build.py
-	$(QUIET_GEN) python3 thirdparty/extract/src/docx_template_build.py \
-		-i thirdparty/extract/src/template.docx \
-		-n docx \
-		-o thirdparty/extract/src/docx_template
+	$(QUIET_GEN) if python3 -c 'import sys; sys.exit(sys.version_info < (3,6))'; \
+		then python3 thirdparty/extract/src/docx_template_build.py \
+			-i thirdparty/extract/src/template.docx \
+			-n docx \
+			-o thirdparty/extract/src/docx_template ; \
+		else echo python3 is older than 3.6 so not running docx_template_build.py; \
+		fi
 	@touch $@
 
 thirdparty/extract/src/odt_template.c: thirdparty/extract/src/template.odt thirdparty/extract/src/docx_template_build.py
-	$(QUIET_GEN) python3 thirdparty/extract/src/docx_template_build.py \
-		-i thirdparty/extract/src/template.odt \
-		-n odt \
-		-o thirdparty/extract/src/odt_template
+	$(QUIET_GEN) if python3 -c 'import sys; sys.exit(sys.version_info < (3,6))'; \
+		then python3 thirdparty/extract/src/docx_template_build.py \
+			-i thirdparty/extract/src/template.odt \
+			-n odt \
+			-o thirdparty/extract/src/odt_template ; \
+		else echo python3 is older than 3.6 so not running docx_template_build.py; \
+		fi
 	@touch $@
 
 generate: thirdparty/extract/src/docx_template.c