prepare ChangeLog for 6.4.0
[test] t/dynoplibs/debug.t - Skip debug_break on darwin also. See GH#1071
Prepare for the 6.4.0 release.
[config] fix failing ICU headers probes on multi-arch

We must not die when perl does not find the icuheaders.
cc needs to find it, and it does it in the probe if they are
in the proper cc searchdir.

icu-config does not store the full headerpath, only pkg-config.
Fixes [GH #1014] on debian/ubuntu
Merge branch 'rurban/icuheaders-gh1014'

Confirmed ok by pmichaud on Kubuntu 14.04, and also tested ok on solaris
[opengl] Fix BSD problem which does not define __APPLE__

Fix opengl on bsd which does not have __APPLE__ defined as 0.
freeglut from 2.0 to at least 2.8 relies on that define.
Fixes [GH #1070].
pmc2c: inline gc write barrier into vtable methods

removed wrong UNUSED(INTERP) and UNUSED(SELF)
some methods with return values are marked TODO, need to add the write barriers manually.

A missing write barrier caused one error:
t/examples/shootout.t: 16 examples/shootout/regexdna.pir SEGV

TODO GC write barrier to:
default.pop_float
default.pop_integer
default.pop_pmc
default.pop_string
default.shift_float
default.shift_integer
default.shift_pmc
default.shift_string
CallContext.shift_pmc
CallContext.shift_string
Key.shift_pmc
ArrayIterator.pop_float
ArrayIterator.pop_integer
ArrayIterator.pop_pmc
ArrayIterator.pop_string
ArrayIterator.shift_float
ArrayIterator.shift_integer
ArrayIterator.shift_pmc
ArrayIterator.shift_string
Task.pop_pmc
Capture.pop_float
Capture.pop_integer
Capture.pop_pmc
Capture.pop_string
Capture.shift_float
Capture.shift_integer
Capture.shift_pmc
Capture.shift_string
HashIterator.shift_integer
HashIterator.shift_pmc
HashIterator.shift_string
ImageIOThaw.shift_float
ImageIOThaw.shift_integer
ImageIOThaw.shift_pmc
ImageIOThaw.shift_string
ResizablePMCArray.pop_float
ResizablePMCArray.pop_integer
ResizablePMCArray.pop_pmc
ResizablePMCArray.pop_string
ResizablePMCArray.shift_float
ResizablePMCArray.shift_integer
ResizablePMCArray.shift_pmc
ResizablePMCArray.shift_string
Object.pop_float
Object.pop_integer
Object.pop_pmc
Object.pop_string
Object.shift_float
Object.shift_integer
Object.shift_pmc
Object.shift_string
OrderedHashIterator.pop_pmc
OrderedHashIterator.shift_pmc
OrderedHashIterator.shift_string
PMCList.pop_pmc
PMCList.shift_pmc
ResizableBooleanArray.pop_integer
ResizableBooleanArray.shift_integer
ResizableFloatArray.pop_float
ResizableFloatArray.shift_float
ResizableIntegerArray.pop_integer
ResizableIntegerArray.shift_integer
ResizableStringArray.pop_float
ResizableStringArray.pop_integer
ResizableStringArray.pop_pmc
ResizableStringArray.pop_string
ResizableStringArray.shift_float
ResizableStringArray.shift_integer
ResizableStringArray.shift_pmc
ResizableStringArray.shift_string
Scheduler.shift_pmc
StringIterator.pop_integer
StringIterator.pop_pmc
StringIterator.pop_string
StringIterator.shift_integer
StringIterator.shift_pmc
StringIterator.shift_string
[pmc2c] WIP :no_wb, detection of :manual_wb, RETURN() also for VTABLE methods

add_write_barrier now calls Parrot::Pmc2c::PCCMETHOD::rewrite_RETURNs, but
this does not work yet.
[pmc] partially use new WB markers
[pmc2c] set manual_wb for :no_wb and PARROT_GC_WRITE_BARRIER in --dump

detect it now earlier, in the parser
[pmc2c] small decl optimization
[codingstd] fix a cuddled else