"Update synctex internal copy to 1.21"

synctex: Update from version 1.18 to 1.21

Version 1.19 changes the API, normalized the data types, and function names.

The major change for Evince is to add an extra field in EvSourceLink and the prototype to create the structure. synctex_display_query now requires an extra parameters (page-hint) to improve the results, but the page has not been store before.

To build using the system SyncTex, we now required at least version 1.19. Thus, we avoid filling the code with #ifdef.

I committed separately the changes I introduced in synctex source code. If you see the commit, you can check if I did something unorthodox or if there is a better solution for this.

@jaliste what do you think?

Closes #878 (closed)