Fix PACKAGE_MANAGER validation in CMakeLists.txt (#309)

Fixes #308

Author: w15eacre

CMakeLists.txt

@@ -1,7 +1,7 @@
 cmake_minimum_required(VERSION 3.12)
 
 if (PACKAGE_MANAGER)
-  if(PACKAGE_MANAGER NOT MATCHES "^(hunter|vcpkg)$")
+  if(NOT PACKAGE_MANAGER MATCHES "^(hunter|vcpkg)$")
     message(FATAL_ERROR "PACKAGE_MANAGER must be set to 'hunter', 'vcpkg' or isn't set")
   endif ()
 else ()