with recent change, OpenSSL download URL was changed in
incompatiable way. i.e. only the most recent openssl version
might be downloaded using previous script.
older versions are available under different URLs. as we need
several openssl versions, let us adopt script accordingly.
bug was caught after travis-ci cache was purged for some reason.
full list:
update LibreSSL to 2.9.2
speed up build by using "make -j3"
cache BoringSSL checkout
build prometeus exporter
add basic cygwin build
add USE_TFO=1, USE_SYSTEMD=1 to linux builds
