CMake: Require libgnutls by default.

- Make cmake complain and abort the build process if libgnutls is not
  available.  In order to build Taskwarrior without "sync" support,
  or to build it on a system with libgnutls missing, the new cmake
  flag "-DENABLE_SYNC=OFF" can be used.

ChangeLog

@@ -1,27 +1,30 @@
 2.4.5 () -
 
 - TW-32   Unable to change annotations via task edit (thanks to Peter De
-Poorter).
+          Poorter).
 - TW-50   Creation time is lost after editing (thanks to Ben Boeckel).
 - TW-148  default.command options (thanks to David Patrick).
 - TW-269  \t is not rendered correctly (thanks to Scott Kostyshak).
 - TW-283  Process rc.* items when executing config (thanks to Ben Armstrong).
 - TW-303  Ability for "import" to update existing tasks (thanks to Kosta
-Harlan).
+          Harlan).
 - TW-1285 relative dates combined with times (thanks to Adam Gibbins).
 - TW-1432 start/stop can be issued on completed tasks  (thanks to Renato Alves).
 - TW-1440 "task import" from STDIN (thanks to Renato Alves).
 - TW-1454 Redundant dependency should not stop modification  (thanks to Tomas
-Babej).
+          Babej).
 - TW-1572 Better urgency inheritance (thanks to Jens Erat).
 - Prevent potential task duplication during import for non-pending tasks.
 - Show the active context in "context list", if any is active.
 - Fix "task edit" dropping annotation text after newlines.
 - Removed obsolete script 'context'.
 - Fix "project" verbosity info not showing without "footnote" being manually
-enabled.
+  enabled.
 - Internal plumbing:  Use variadic templates for format(), reducing code bloat
-and enabling more flexible use of the function.
+  and enabling more flexible use of the function.
+- Enable "task sync" support by default.  "cmake -DENABLE_SYNC=OFF" allows
+  disabling it and building Taskwarrior without libgnutls available.
+
 
 ------ current release ---------------------------