curl needs -L to follow redirects when downloading

If the build system uses curl to download protobuf, it gets stuck on a redirect. It looks like curl -L should fix that.

make[3]: Entering directory '/home/muscle3/libmuscle/cpp/build/protobuf'
curl https://github.com/protocolbuffers/protobuf/releases/download/v3.7.1/protobuf-cpp-3.7.1.tar.gz
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100   636  100   636    0     0   1242      0 --:--:-- --:--:-- --:--:--  1244
<html><body>You are being <a href="https://github-production-release-asset-2e65be.s3.amazonaws.com/23357588/3e209400-507c-11e9-9f78-69acbcd594df?X-Amz-Algorithm=AWS4-HMAC-SHA256&amp;X-Amz-Credential=AKIAIWNJYAX4CSVEH53A%2F20200416%2Fus-east-1%2Fs3%2Faws4_request&amp;X-Amz-Date=20200416T113234Z&amp;X-Amz-Expires=300&amp;X-Amz-Signature=4da9ec664d1d2a6cf3511141c2974d787d190305cb78459f0cd178501e1f6f48&amp;X-Amz-SignedHeaders=host&amp;actor_id=0&amp;repo_id=23357588&amp;response-content-disposition=attachment%3B%20filename%3Dprotobuf-cpp-3.7.1.tar.gz&amp;response-content-type=application%2Foctet-stream">redirected</a>.</body></html>tar xf protobuf-cpp-3.7.1.tar.gz
tar: protobuf-cpp-3.7.1.tar.gz: Cannot open: No such file or directory
tar: Error is not recoverable: exiting now

multiscale / muscle3

curl needs -L to follow redirects when downloading #46