I came across this issue, this morning, and thought I would pass along a solution. I was trying to install Asterisk v16 on Ubuntu 18, from source. Everything seemed to go fine, until the configure stage, where is was complaining that libxml2-dev was not installed, but libxml2-dev would not install, because of a version issue in libpcre3 & libpcre16 Basically, I had to force apt to install an older version, using the following command.
apt-get install libpcre3=2:8.39-9 libpcre3-dev=2:8.39-9 libpcre16-3=2:8.39-9
Then, install libxml2-dev.
apt-get install libxml2-dev