CMake: set reasonable default value for CMAKE_INSTALL_PREFIX

Author: TheMostDiligent

CMakeLists.txt

@@ -429,9 +429,12 @@ file(RELATIVE_PATH DILIGENT_CORE_DIR "${CMAKE_SOURCE_DIR}" "${CMAKE_CURRENT_SOUR
 SET(DILIGENT_CORE_DIR ${DILIGENT_CORE_DIR} CACHE INTERNAL "Diligent Core installation directory")
 
 SET(DILIGENT_CORE_INSTALL_LIBS_LIST "" CACHE INTERNAL "Core libraries installation list")
-# CMAKE_INSTALL_PREFIX must be absolute otherwise rpath won't work
-if(NOT IS_ABSOLUTE ${CMAKE_INSTALL_PREFIX})
-    set(CMAKE_INSTALL_PREFIX "${CMAKE_BINARY_DIR}/${CMAKE_INSTALL_PREFIX}" CACHE FILEPATH "Installation path" FORCE)
+
+if (CMAKE_INSTALL_PREFIX_INITIALIZED_TO_DEFAULT)
+    set(CMAKE_INSTALL_PREFIX "${CMAKE_BINARY_DIR}/install" CACHE PATH "Installation path" FORCE)
+elseif (NOT IS_ABSOLUTE ${CMAKE_INSTALL_PREFIX})
+    # CMAKE_INSTALL_PREFIX must be absolute otherwise rpath won't work
+    set(CMAKE_INSTALL_PREFIX "${CMAKE_BINARY_DIR}/${CMAKE_INSTALL_PREFIX}" CACHE PATH "Installation path" FORCE)
     message("Transformed CMAKE_INSTALL_PREFIX into absolute path: " ${CMAKE_INSTALL_PREFIX})
 endif()