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.

CMakeLists.txt

@@ -12,7 +12,14 @@ set (HAVE_CMAKE true)
 project (task)
 set (PROJECT_VERSION "2.4.5")
 
-OPTION(USE_GNUTLS "Build gnutls support." ON)
+OPTION (ENABLE_SYNC "Enable 'task sync' support" ON)
+
+if (ENABLE_SYNC)
+  set (USE_GNUTLS ON CACHE BOOL "Build gnutls support." FORCE)
+else (ENABLE_SYNC)
+  set (USE_GNUTLS OFF CACHE BOOL "Build gnutls support." FORCE)
+  message (WARNING "ENABLE_SYNC=OFF. Not building sync support.")
+endif (ENABLE_SYNC)
 
 message ("CMAKE_SYSTEM_NAME ${CMAKE_SYSTEM_NAME}")
 
@@ -116,6 +123,10 @@ if (USE_GNUTLS)
    endif (GNUTLS_FOUND)
 endif (USE_GNUTLS)
 
+if (ENABLE_SYNC AND NOT GNUTLS_FOUND)
+  message (FATAL_ERROR "Cannot find GnuTLS. Use -DENABLE_SYNC=OFF to build Taskwarrior without sync support. See INSTALL for more information.")
+endif (ENABLE_SYNC AND NOT GNUTLS_FOUND)
+
 check_function_exists (timegm  HAVE_TIMEGM)
 check_function_exists (get_current_dir_name HAVE_GET_CURRENT_DIR_NAME)
 check_function_exists (wordexp HAVE_WORDEXP)