OpenCVConfig.cmake: Fix CMake CMP0012 warning

"TRUE"/"FALSE" should not be used as literals, as CMake has to decide
whether they are literals or variable names (sigh).
This patch should make it work with any version of CMake and with any
user-side decision on old vs. new policy behavior.

cmake/templates/OpenCVConfig.cmake.in

@@ -49,7 +49,7 @@ if(NOT DEFINED OpenCV_MODULES_SUFFIX)
   endif()
 endif()
 
-if(@USE_IPPICV@) # value is defined by package builder
+if("@USE_IPPICV@" STREQUAL "TRUE") # value is defined by package builder (use STREQUAL to comply new CMake policy CMP0012)
   if(NOT TARGET ippicv)
     if(EXISTS "${CMAKE_CURRENT_LIST_DIR}/@INSTALL_PATH_RELATIVE_IPPICV@")
       add_library(ippicv STATIC IMPORTED)