Consolidate and simplify CMAKE_POLICY entries (#6780)

Simplify CMake Policies
Per https://github.com/LMMS/lmms/pull/6758#discussion_r1271346366

cmake/modules/DefineInstallVar.cmake

@@ -24,7 +24,7 @@ function(DEFINE_INSTALL_VAR)
         endif()
     else()
         if(VAR_GENERATOR_EXPRESSION)
-            cmake_policy(SET CMP0087 NEW)
+            cmake_policy(SET CMP0087 NEW) # install(CODE) and install(SCRIPT) support generator expressions.
         endif()
         install(CODE "set(\"${VAR_NAME}\" \"${VAR_CONTENT}\")")
     endif()

cmake/modules/InstallDependencies.cmake

@@ -1,8 +1,8 @@
 include(GetPrerequisites)
 include(CMakeParseArguments)
 
-CMAKE_POLICY(SET CMP0011 NEW)
-CMAKE_POLICY(SET CMP0057 NEW)
+# Project's cmake_minimum_required doesn't always propagate
+cmake_policy(SET CMP0057 NEW) # Support new if() IN_LIST operator.
 
 function(make_absolute var)
 	get_filename_component(abs "${${var}}" ABSOLUTE BASE_DIR "${CMAKE_INSTALL_PREFIX}")