Replace the impenetrable line of perl with python

Mostly because the standard emscripten docker image does not have
libjson-perl, but python always comes with json. But also because
it was impenetrable.

Makefile

@@ -164,7 +164,7 @@ fuzzers: $(FUZZER_BINARIES) $(FUZZER_DEBUG_BINARIES)
 .PHONY: fuzzers
 
 $(JS_EXPORTED_FUNCTIONS): $(PUBLIC_HEADERS)
-	perl -MJSON -ne '$$f{"_$$1"}=1 if /(olm_[^( ]*)\(/; END { @f=sort keys %f; print encode_json \@f }' $^ > $@.tmp
+	./exports.py $^ > $@.tmp
 	mv $@.tmp $@
 
 all: test js lib debug doc

exports.py

@@ -0,0 +1,18 @@
+#!/usr/bin/env python
+
+import sys
+import re
+import json
+
+expr = re.compile(r"(olm_[^( ]*)\(")
+
+exports = set()
+
+for f in sys.argv[1:]:
+    with open(f) as fp:
+        for line in fp:
+            matches = expr.search(line)
+            if matches is not None:
+                exports.add('_%s' % (matches.group(1),))
+
+json.dump(sorted(exports), sys.stdout)