OpenSSL not found during ./configure

You will find a package name libssl-dev in Synaptic Package Manager (Ubuntu 16.04) and after you install libssl-dev, everything will be solved. by the way, libssl1.0.0 don't works. You'd better install libssl-dev and libssl1.0.0 at same time.

by the way, I don't think the following solution is works: 'pkg-config' is in your PATH and PKG_CONFIG_PATH contains a openssl.pc file. If pkg-config is unavailable you can set INCLUDE_DIR and LIB_DIR manually via: R CMD INSTALL --configure-vars='INCLUDE_DIR=... LIB_DIR=...'

For some reason PKG_CONFIG_PATH didn't contain the right path for that openssl.pc file needed for pkg-config utility. I did find / -name 'openssl.pc' and found it in /usr/lib/x86_64-linux-gnu/pkgconfig/. That path was also used by my other ubuntu to ./configure.

So I ran PKG_CONFIG_PATH="/usr/lib/x86_64-linux-gnu/pkgconfig/" ./configure and that did the trick!

Other thing, there is another openssl.pc in my /usr/local/ssl/lib/pkgconfig/ but when I tried to use it, the following make command hangs with different errors (like unknown reference to function, etc.).