CMake: Disable variable expansion for quoted strings in if()

This makes the build system more rubust against stray variables
coming from included CMake scripts provided by the system.

Fixes build with CMake 3.5 (tested with 3.5.0-rc3).

Fixes: issue #50

CMakeLists.txt

@@ -2,6 +2,11 @@ project(innoextract)
 
 cmake_minimum_required(VERSION 2.8)
 
+if(POLICY CMP0054)
+	# CMake 3.1+: Only interpret if() arguments as variables or keywords when unquoted.
+	cmake_policy(SET CMP0054 NEW)
+endif()
+
 
 # Define configuration options