pthread component needs "newlib_include_locks_impl" type treatment for gcc 8 (IDFGH-1437)

[bpietsch]

I'm using c++ and the gthread library has a dependency on pthread_cond_xxx. I found that with gcc 8.2 I had to apply the following bit of trickery (copied from newlib) to be able to link (along with corresponding stub functions in the source files, of course):

if(GCC_NOT_5_2_0)
set(EXTRA_LINK_FLAGS "-u pthread_include_pthread_impl")
list(APPEND EXTRA_LINK_FLAGS "-u pthread_include_pthread_cond_impl")
list(APPEND EXTRA_LINK_FLAGS "-u pthread_include_pthread_local_storage_impl")
endif()

idf_component_register(SRCS "pthread.c"
"pthread_cond_var.c"
"pthread_local_storage.c"
INCLUDE_DIRS include)

if(CONFIG_FREERTOS_ENABLE_STATIC_TASK_CLEAN_UP)
target_link_libraries(${COMPONENT_LIB} "-Wl,--wrap=vPortCleanUpTCB")
endif()

if(EXTRA_LINK_FLAGS)
target_link_libraries(${COMPONENT_LIB} INTERFACE "${EXTRA_LINK_FLAGS}")
endif()