GCC 4.4, the minimum compiler version, supports this option. Unlike
other warnings, -Wimplicit-function-declaration warnings should be
independent of compiler versions, so this change should not cause
compiler-specific build failures.
This feature is specifically for the C++ compiler to offload calling
thread_local object destructors on thread program exit, to glibc.
This is to overcome the possible complication of destructors of
thread_local objects getting called after the DSO in which they're
defined is unloaded by the dynamic linker. The DSO is marked as
'unloadable' if it has a constructed thread_local object and marked as
'unloadable' again when all the constructed thread_local objects
defined in it are destroyed.
