cmake: Avoid adding default path as system directory for GCC 6.x

This is a completion for PR #7390 and fixes #6517 and #815.

cmake/OpenCVPCHSupport.cmake

@@ -69,6 +69,9 @@ MACRO(_PCH_GET_COMPILE_FLAGS _out_compile_flags)
     FOREACH(item ${DIRINC})
         if(item MATCHES "^${OpenCV_SOURCE_DIR}/modules/")
           LIST(APPEND ${_out_compile_flags} "${_PCH_include_prefix}\"${item}\"")
+        elseif(CMAKE_COMPILER_IS_GNUCXX AND NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS "6.0" AND
+               item MATCHES "/usr/include$")
+          # workaround for GCC 6.x bug
         else()
           LIST(APPEND ${_out_compile_flags} "${_PCH_isystem_prefix}\"${item}\"")
         endif()
@@ -78,6 +81,9 @@ MACRO(_PCH_GET_COMPILE_FLAGS _out_compile_flags)
     FOREACH(item ${DIRINC})
         if(item MATCHES "^${OpenCV_SOURCE_DIR}/modules/")
           LIST(APPEND ${_out_compile_flags} "${_PCH_include_prefix}\"${item}\"")
+        elseif(CMAKE_COMPILER_IS_GNUCXX AND NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS "6.0" AND
+               item MATCHES "/usr/include$")
+          # workaround for GCC 6.x bug
         else()
           LIST(APPEND ${_out_compile_flags} "${_PCH_isystem_prefix}\"${item}\"")
         endif()