check for JSON perl module and if not found ask developer to install it

partial fix for #74

autogen.sh

@@ -5,3 +5,5 @@
 set -e
 autoreconf -f -i || { echo "autoreconf failed: $?"; exit 1; }
 find . \( -name '*-private.h' -o -name '*-protos.h' \) | xargs rm -f
+perl -MJSON -e 'print foo;' || \
+    { echo "you must install JSON perl module (cpan install JSON)"; exit 1; }