cmake: fix pkg-config handling

backporting commit c9f3f4d1

CMakeLists.txt

@@ -719,7 +719,7 @@ ocv_cmake_hook(POST_COMPILER_OPTIONS)
 # ----------------------------------------------------------------------------
 if(UNIX)
   if(NOT APPLE_FRAMEWORK OR OPENCV_ENABLE_PKG_CONFIG)
-    if(CMAKE_CROSSCOMPILING AND NOT DEFINED ENV{PKG_CONFIG_LIBDIR}
+    if(CMAKE_CROSSCOMPILING AND NOT DEFINED ENV{PKG_CONFIG_LIBDIR} AND NOT DEFINED ENV{PKG_CONFIG_SYSROOT_DIR}
         AND NOT OPENCV_ENABLE_PKG_CONFIG
     )
       if(NOT PkgConfig_FOUND)

cmake/OpenCVUtils.cmake

@@ -743,7 +743,7 @@ macro(ocv_check_modules define)
     endif()
     unset(${define}_${__modname}_FOUND)
   endforeach()
-  if(COMMAND pkg_check_modules)
+  if(PKG_CONFIG_FOUND OR PkgConfig_FOUND)
     pkg_check_modules(${define} ${ARGN})
   endif()
   if(${define}_FOUND)