STYLE: Simplify top-level CMakeLists removing LINUX ivar

The variable is currently used only in Wrappers project and was originally
introduced in 6acfa41f (COMP: Build audio only on windows).

CMakeLists.txt

 cmake_minimum_required(VERSION 3.9)
 project(iMSTK VERSION 5.0.0 LANGUAGES C CXX)
 
-if(UNIX AND NOT APPLE)
-  set(LINUX TRUE)
-endif()
-
 #-----------------------------------------------------------------------------
 # Set a default build type if none was specified
 #-----------------------------------------------------------------------------

Source/Wrappers/CMakeLists.txt

@@ -85,7 +85,7 @@ install( TARGETS iMSTKCWrapper EXPORT iMSTKCWrapperTargets
     LIBRARY DESTINATION lib COMPONENT RuntimeLibraries
     ARCHIVE DESTINATION lib COMPONENT Development)
 
-if (LINUX)
+if (UNIX AND NOT APPLE)
 	# Why use CODE and then execute COMMAND?
     install(CODE "execute_process(COMMAND ${CMAKE_COMMAND} -E create_symlink iMSTKCWrapper.so ${CMAKE_INSTALL_PREFIX}/lib/libiMSTKCWrapper.so)")
 endif()