cmake: don't use absolute paths with/without CMAKE_INSTALL_PREFIX

CPack can't work with absolute paths.

CMakeLists.txt

@@ -386,7 +386,7 @@ else()
     ocv_update(OPENCV_SAMPLES_SRC_INSTALL_PATH    samples/native)
     ocv_update(OPENCV_JAR_INSTALL_PATH java)
     ocv_update(OPENCV_OTHER_INSTALL_PATH   etc)
-    ocv_update(OPENCV_CONFIG_INSTALL_PATH  "")
+    ocv_update(OPENCV_CONFIG_INSTALL_PATH  ".")
   else()
     ocv_update(OPENCV_LIB_INSTALL_PATH     lib${LIB_SUFFIX})
     ocv_update(OPENCV_3P_LIB_INSTALL_PATH  share/OpenCV/3rdparty/${OPENCV_LIB_INSTALL_PATH})

cmake/OpenCVGenConfig.cmake

@@ -79,7 +79,7 @@ function(ocv_gen_config TMP_DIR NESTED_PATH ROOT_NAME)
   install(FILES
       "${TMP_DIR}/OpenCVConfig-version.cmake"
       "${__tmp_nested}/OpenCVConfig.cmake"
-      DESTINATION "${CMAKE_INSTALL_PREFIX}/${__install_nested}" COMPONENT dev)
+      DESTINATION "${__install_nested}" COMPONENT dev)
 
   if(ROOT_NAME)
     # Root config file
@@ -87,7 +87,7 @@ function(ocv_gen_config TMP_DIR NESTED_PATH ROOT_NAME)
     install(FILES
         "${TMP_DIR}/OpenCVConfig-version.cmake"
         "${TMP_DIR}/OpenCVConfig.cmake"
-        DESTINATION "${CMAKE_INSTALL_PREFIX}/${OPENCV_CONFIG_INSTALL_PATH}" COMPONENT dev)
+        DESTINATION "${OPENCV_CONFIG_INSTALL_PATH}" COMPONENT dev)
   endif()
 endfunction()
 

cmake/OpenCVUtils.cmake

@@ -87,7 +87,11 @@ endmacro()
 macro(ocv_path_join result_var P1 P2)
   string(REGEX REPLACE "^[/]+" "" P2 "${P2}")
   if("${P1}" STREQUAL "")
-    set(${result_var} "${P2}")
+    if("${P2}" STREQUAL "")
+      set(${result_var} ".")
+    else()
+      set(${result_var} "${P2}")
+    endif()
   elseif("${P1}" STREQUAL "/")
     set(${result_var} "/${P2}")
   elseif("${P2}" STREQUAL "")