Prevent invoking Jinja templating unecessarily

Makefile

@@ -54,10 +54,10 @@ perftest: compile_all
 	$(BINDIR)/sharp_testsuite test gauss 4095 -1 -1 8192 0 1 && \
 	$(BINDIR)/sharp_testsuite test gauss 8191 -1 -1 16384 0 1
 
-# Jinja templates
-
 %.c: %.c.in
-	./runjinja.py < $< > $@
+# Only do this if the md5sum changed, in order to avoid Python and Jinja
+# dependency when not modifying the c.in file
+	grep `md5sum $< | cut -d ' ' -f 1` $@ || ./runjinja.py < $< > $@
 
 genclean:
 	rm libsharp/sharp_legendre.c || exit 0

libsharp/sharp_legendre.c

@@ -1,4 +1,4 @@
-/*
+/* DO NOT EDIT. md5sum of source: 88eec944ab6fbfdc7f391fbb58df3bf1 *//*
 
     NOTE NOTE NOTE
 
@@ -1303,3 +1303,5 @@ void sharp_legendre_transform_s(float *bl,
     }
 }
 
+
+

libsharp/sharp_legendre.c.in

@@ -160,4 +160,3 @@ void sharp_legendre_transform{{T}}({{scalar}} *bl,
     }
 }
 /*{ endfor }*/
-

runjinja.py

@@ -5,6 +5,7 @@ Preprocesses foo.c.in to foo.c. Reads STDIN and writes STDOUT.
 """
 
 import sys
+import hashlib
 from jinja2 import Template, Environment
 
 env = Environment(block_start_string='/*{',
@@ -13,4 +14,6 @@ env = Environment(block_start_string='/*{',
                   variable_end_string='}}')
 
 extra_vars = dict(len=len)
-sys.stdout.write(env.from_string(sys.stdin.read()).render(**extra_vars))
+input = sys.stdin.read()
+sys.stdout.write('/* DO NOT EDIT. md5sum of source: %s */' % hashlib.md5(input).hexdigest())
+sys.stdout.write(env.from_string(input).render(**extra_vars))