Fix optimized mingw/gcc-8.2 builds

gcc-8.2 -O3 optimization enables -finline-functions.
For reasons yet unknown this causes issues with Glib::Threads::Private
when loading sessions (creating new ones is fine) and likely some
other operations.

Without this change the following happens (5.12 as well as current git)
when loading an existing session:

---
[Switching to Thread 6092.0x12fc]
Breakpoint 1, ARDOUR::SessionEvent::create_per_thread_pool (name=..., nitems=4096)
    at ../libs/ardour/session_events.cc:60
60      in ../libs/ardour/session_events.cc
$6 = (const std::__cxx11::string &) @0x210dfc60: {static npos = 18446744073709551615,
  _M_dataplus = {<std::allocator<char>> = {<__gnu_cxx::new_allocator<char>> = {<No data fields>}, <No data fields>},
    _M_p = 0x210dfc70 "GUI"}, _M_string_length = 3, {_M_local_buf = "GUI\000\000\000\000\000<E0>P<DA>:\000\000\000",
    _M_allocated_capacity = 4805959}}
#0  ARDOUR::SessionEvent::create_per_thread_pool (name=..., nitems=4096) at ../libs/ardour/session_events.cc:60
#1  0x0000000000434e77 in ?? ()

[..]

Thread 1 (Thread 6092.0x12fc):
#0  0x0000000061f152ef in PBD::RingBuffer<void*>::write (this=this@entry=0x50, src=src@entry=0x210dcdc8,
    cnt=cnt@entry=1) at ../libs/pbd/pbd/ringbuffer.h:170
#1  0x0000000061ee954d in push (t=<optimized out>, this=0x0) at ../libs/pbd/pool.cc:290
#2  CrossThreadPool::flush_pending_with_ev (this=0x0, ptr=<optimized out>) at ../libs/pbd/pool.cc:254
#3  0x00000000218d5b01 in operator() (a0=0x210dcdc8, this=0x48)
    at /home/ardour/win-stack-w64/include/boost/function/function_template.hpp:682
#4  operator()<boost::function<void(ARDOUR::SessionEvent*)>, boost::_bi::list0> (a=<synthetic pointer>, f=...,
    this=0x68) at /home/ardour/win-stack-w64/include/boost/bind/bind.hpp:259
#5  operator() (this=0x48) at /home/ardour/win-stack-w64/include/boost/bind/bind.hpp:1294
#6  boost::detail::function::void_function_obj_invoker0<boost::_bi::bind_t<boost::_bi::unspecified, boost::function<void (ARDOUR::SessionEvent*)>, boost::_bi::list1<boost::_bi::value<ARDOUR::SessionEvent*> > >, void>::invoke(boost::detail::function::function_buffer&) (function_obj_ptr=...)
    at /home/ardour/win-stack-w64/include/boost/function/function_template.hpp:159
#7  0x0000000063e2dc39 in operator() (this=0x210dcf70)
    at /home/ardour/win-stack-w64/include/boost/function/function_template.hpp:682
#8  AbstractUI<Gtkmm2ext::UIRequest>::call_slot(PBD::EventLoop::InvalidationRecord*, boost::function<void ()> const&)
    (this=0x3ada50e0, invalidation=0x0, f=...) at ../libs/pbd/pbd/abstract_ui.cc:434
#9  0x00000000216cbb96 in ARDOUR::SessionEventManager::merge_event (this=<optimized out>, ev=<optimized out>)
    at /home/ardour/win-stack-w64/include/boost/function/function_template.hpp:507
#10 0x00000000216e3861 in non-virtual thunk to ARDOUR::Session::queue_event(ARDOUR::SessionEvent*) ()
    at ../../win-stack-w64/include/glibmm-2.4/glibmm/threads.h:709
#11 0x00000000216c91bf in ARDOUR::SessionEventManager::clear_events(ARDOUR::SessionEvent::Type, boost::function<void ()>) (this=this@entry=0x4256cc08, type=type@entry=ARDOUR::SessionEvent::Skip, after=...)
    at ../libs/ardour/session_events.cc:156
#12 0x000000002168874a in ARDOUR::Session::sync_locations_to_skips (this=this@entry=0x4256ca90)
    at /usr/lib/gcc/x86_64-w64-mingw32/8.3-posix/include/c++/new:169
#13 0x0000000021688802 in ARDOUR::Session::update_skips (this=this@entry=0x4256ca90, loc=loc@entry=0x0,
    consolidate=consolidate@entry=false) at ../libs/ardour/session.cc:1683
#14 0x0000000021699649 in _locations_changed (locations=..., this=0x4256ca90) at ../libs/ardour/session.cc:1859
#15 apply<ARDOUR::Session> (method=
    (void (ARDOUR::Session::*)(ARDOUR::Session * const, const ARDOUR::Locations::LocationList &)) 0x216994f0 <ARDOUR::Session::_locations_changed(std::__cxx11::list<ARDOUR::Location*, std::allocator<ARDOUR::Location*> > const&)>,
    obj=..., this=0x40ff4c60) at ../libs/ardour/ardour/location.h:240
#16 ARDOUR::Session::locations_changed (this=this@entry=0x4256ca90) at ../libs/ardour/session.cc:1840
#17 0x0000000021700abd in ARDOUR::Session::set_state (this=0x4256ca90, node=..., version=6000)
    at ../libs/ardour/session_state.cc:1657
#18 0x0000000021713456 in ARDOUR::Session::post_engine_init (this=this@entry=0x4256ca90) at ../libs/pbd/pbd/xml++.h:81
#19 0x00000000216ad5cd in ARDOUR::Session::Session (this=0x4256ca90, eng=..., fullpath=..., snapshot_name=...,
    bus_profile=<optimized out>, mix_template=...) at ../libs/ardour/session.cc:383
#20 0x000000000046db8b in ?? ()
Backtrace stopped: previous frame inner to this frame (corrupt stack?)

---

Note that in the trace starting at #19 the source-file references
are offset :post_engine_init() isn't in xml++.h
(likely debug symbols are incorrect due to inline functions)

The application crashes due to #2 ::flush_pending_with_ev (this=0x0 ..)
But the earlier breakpoint above shows that the ThreadPool was
created, and this #11 ::clear_events() should not assign NULL.

(This happens on a single-core CPU, too)

wscript

@@ -549,6 +549,7 @@ int main() { return 0; }''',
 
                 # mingw/gcc-8.2
                 compiler_flags.append(flags_dict['xsaveintrin'])
+                compiler_flags.append('-fno-inline-functions')
 
     # end of processor-specific section