wx: Only call XInitThreads for linux

Turns out OSX also returns 'posix' for os.name. This limits it to
linux only. Note that it's purely cosmetic because on OSX it would
just print a warning but not prevent anything from working.

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>
wxgui: Make sure to only start the flow graph once all init is done

Currently we start the flow graph in the stdframe constructor.
However at that point, the frame isn't attached to the app and it's not
"shown" yet which means some UI operations can still fail.

This is a race condition which happens on OSX. You can reproduce it on
linux as well by adding a sleep(1) after starting the flow graph.

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>
volk: adding volk_malloc and volk_free functions to handle allocation of aligned memory.
filter: switched from using fft::malloc to volk_malloc.
qtgui: switched from using fft::malloc to volk_malloc.