please adjust URL in OpenBSD build instructions

[plavcik]

in doc/build-openbsd.md

is download for boost_1_61_0.tar.bz2 referenced to a non working URL http://heanet.dl.sourceforge.net/project/boost/boost/1.61.0/boost_1_61_0.tar.bz2

I successfully downloaded and SHA256 checked boost_1_61_0.tar.bz2 from this URL
https://downloads.sf.net/project/boost/boost/1.61.0/boost_1_61_0.tar.bz2

(that URL is adjusted to some mirror, which is ok)

thank you

[laanwj]

I have an (unsubmitted) patch that updates boost version and URI, might be helpful:

-curl -o boost_1_61_0.tar.bz2 http://heanet.dl.sourceforge.net/project/boost/boost/1.61.0/boost_1_61_0.tar.bz2
-echo 'a547bd06c2fd9a71ba1d169d9cf0339da7ebf4753849a8f7d6fdb8feee99b640  boost_1_61_0.tar.bz2' | sha256 -c
-# MUST output: (SHA256) boost_1_61_0.tar.bz2: OK
-tar -xjf boost_1_61_0.tar.bz2
+curl -o boost_1_63_0.tar.bz2 https://netcologne.dl.sourceforge.net/project/boost/boost/1.63.0/boost_1_63_0.tar.bz2
+echo 'beae2529f759f6b3bf3f4969a19c2e9d6f0c503edcb2de4a61d1428519fcb3b0 boost_1_63_0.tar.bz2' | sha256 -c
+# MUST output: (SHA256) boost_1_63_0.tar.bz2: OK
+tar -xjf boost_1_63_0.tar.bz2

-# Boost 1.61 needs one small patch for OpenBSD
-cd boost_1_61_0
+# Boost 1.63 needs one small patch for OpenBSD
+cd boost_1_63_0