U++

I created some possible list of plugins to update (with links):
uppsrc/plugin/bz2:
http://www.bzip.org/downloads.html
http://www.bzip.org/1.0.6/bzip2-1.0.6.tar.gz

uppsrc/plugin/glew:
http://glew.sourceforge.net/
http://glew.sourceforge.net/log.html
https://sourceforge.net/projects/glew/files/glew/
https://sourceforge.net/projects/glew/files/glew/2.0.0/glew-2.0.0.tgz/download

uppsrc/plugin/jpg:
http://www.ijg.org/
http://www.ijg.org/files/jpegsrc.v9b.tar.gz

uppsrc/plugin/lz4
http://www.lz4.org
https://github.com/lz4/lz4
https://github.com/lz4/lz4/releases/tag/v1.7.5
https://github.com/lz4/lz4/archive/v1.7.5.tar.gz

uppsrc/plugin/lzma:
http://www.7-zip.org/sdk.html
v16.04:
http://www.7-zip.org/a/lzma1604.7z

uppsrc/plugin/pcre
http://www.pcre.org/
https://ftp.pcre.org/pub/pcre/pcre-8.40.zip
https://ftp.pcre.org/pub/pcre/pcre2-10.22.zip

uppsrc/plugin/png:
http://www.libpng.org/pub/png/libpng.html
https://sourceforge.net/projects/libpng/files/libpng16/1.6.28/
http://download.sourceforge.net/libpng/libpng-1.6.28.tar.gz
https://sourceforge.net/projects/libpng/files/libpng12/1.2.57/
http://download.sourceforge.net/libpng/libpng-1.2.57.tar.gz

uppsrc/plugin/sqlite3:
https://www.sqlite.org/
https://www.sqlite.org/download.html
https://www.sqlite.org/releaselog/3_16_2.html
https://www.sqlite.org/2017/sqlite-amalgamation-3160200.zip

uppsrc/plugin/tif:
http://www.libtiff.org/
http://www.libtiff.org/v3.6.1.html
http://dl.maptools.org/dl/libtiff/
http://dl.maptools.org/dl/libtiff/tiff-3.8.2.tar.gz

uppsrc/plugin/z:
http://zlib.net/
http://zlib.net/ChangeLog.txt
http://zlib.net/zlib-1.2.11.tar.gz

uppsrc/plugin/zstd:
http://facebook.github.io/zstd/
https://github.com/facebook/zstd
https://github.com/facebook/zstd/releases/tag/v1.1.2
https://github.com/facebook/zstd/archive/v1.1.2.tar.gz

Not sure about uppsrc/plugin/tif, because of different versions on home page and download page.