jansson is in Debian and Fedora:

   https://packages.debian.org/source/stretch/jansson
   https://apps.fedoraproject.org/packages/jansson

My cygport file is attached.

Package files:

http://sanibeltranquility.com/cygwin/x86/release/jansson/jansson-2.11-1-src.tar.xz
http://sanibeltranquility.com/cygwin/x86/release/jansson/jansson-2.11-1.hint
http://sanibeltranquility.com/cygwin/x86/release/jansson/libjansson-devel/libjansson-devel-2.11-1.hint
http://sanibeltranquility.com/cygwin/x86/release/jansson/libjansson-devel/libjansson-devel-2.11-1.tar.xz
http://sanibeltranquility.com/cygwin/x86/release/jansson/libjansson-doc/libjansson-doc-2.11-1.hint
http://sanibeltranquility.com/cygwin/x86/release/jansson/libjansson-doc/libjansson-doc-2.11-1.tar.xz
http://sanibeltranquility.com/cygwin/x86/release/jansson/jansson-debuginfo/jansson-debuginfo-2.11-1.hint
http://sanibeltranquility.com/cygwin/x86/release/jansson/jansson-debuginfo/jansson-debuginfo-2.11-1.tar.xz
http://sanibeltranquility.com/cygwin/x86/release/jansson/libjansson4/libjansson4-2.11-1.hint
http://sanibeltranquility.com/cygwin/x86/release/jansson/libjansson4/libjansson4-2.11-1.tar.xz
http://sanibeltranquility.com/cygwin/x86_64/release/jansson/jansson-2.11-1-src.tar.xz
http://sanibeltranquility.com/cygwin/x86_64/release/jansson/jansson-2.11-1.hint
http://sanibeltranquility.com/cygwin/x86_64/release/jansson/libjansson-devel/libjansson-devel-2.11-1.hint
http://sanibeltranquility.com/cygwin/x86_64/release/jansson/libjansson-devel/libjansson-devel-2.11-1.tar.xz
http://sanibeltranquility.com/cygwin/x86_64/release/jansson/libjansson-doc/libjansson-doc-2.11-1.hint
http://sanibeltranquility.com/cygwin/x86_64/release/jansson/libjansson-doc/libjansson-doc-2.11-1.tar.xz
http://sanibeltranquility.com/cygwin/x86_64/release/jansson/jansson-debuginfo/jansson-debuginfo-2.11-1.hint
http://sanibeltranquility.com/cygwin/x86_64/release/jansson/jansson-debuginfo/jansson-debuginfo-2.11-1.tar.xz
http://sanibeltranquility.com/cygwin/x86_64/release/jansson/libjansson4/libjansson4-2.11-1.hint
http://sanibeltranquility.com/cygwin/x86_64/release/jansson/libjansson4/libjansson4-2.11-1.tar.xz

License: MIT

Ken