cmake error if no valac found

[Cris Dywan]

Trying to build on a system with no valac symlink results in a lot of cmake errors with no clear work-around. The VALAC variable has been added recently, it should be mentioned and fail cleanly.
Below the verbatim errors.

CMake Error at cmake/FindVala.cmake:6 (string):
 string sub-command REPLACE requires at least four arguments.
Call Stack (most recent call first):
 CMakeLists.txt:41 (find_package)

CMake Error at cmake/FindVala.cmake:7 (string):
 string sub-command STRIP requires two arguments.
Call Stack (most recent call first):
 CMakeLists.txt:41 (find_package)

CMake Error at cmake/FindVala.cmake:13 (if):
 if given arguments:

 "VERSION_GREATER" "0.16.0" "OR" "VERSION_EQUAL" "0.16.0"

 Unknown arguments specified
Call Stack (most recent call first):
 CMakeLists.txt:42 (vala_require)

-- Configuring incomplete, errors occurred!