More documentation might be found at https://github.com/FreeRDP/FreeRDP/wiki/Compilation

Starting with "El Capitan" Apple removed the openssl headers. Therefore it's
required to build openssl manually upfront. For example by using MacPorts or Homebrew.
To build FreeRDP against this library it's required to set the PKG_CONFIG_PATH
pointing to the openssl root directory befor building.
For example with brew it would look like this:

export PKG_CONFIG_PATH=$(brew --prefix)/opt/openssl/lib/pkgconfig