Is Synergy free if you compile it yourself?
Yes, it is free as it is under the GNU General Public License.
Released under the terms of the GNU General Public License, Synergy is free software.
Note:
- You don't strictly have to compile it yourself, someone else can build it for you, and the developers cannot restrict people sharing built binaries as long as it is GPL licensed.
The paid versions you refer to in the question are for pre-compiled binaries as Synergy charges for distributing these on their website:
On 8 September 2014, the Synergy developers started charging a fee for distribution of pre-compiled binary files of Synergy on their website, while offering a free download for the source code. The developers claim only "0.002% of people were donating" to fund the development before charging.
Source Synergy (software)
Will it have all the features?
Yes, assuming you compile a build with all the features enabled.
Most open-source software comes with build files that enable you to enable/disable specific features where this is applicable.