t/issue27.t improve TODO message and untodo test 1 <5.18
C + test.pl: avoid some compile-time warnings
C: unify empty PVs to ptr_undef

do not manually handle NULL vs &PL_sv_undef as this is already defined by ptr_undef.
ptr_undef can always be initialized statically, so we can remove the init handling.
This fixes #269 with threaded perls, the PV being initialized to &PL_sv_undef, which
cannot be realloced.
t/test.pl: fix /usr/lib/libperl.so linkage

debian (at least) prefers /usr/lib/libperl.so over any given -L path, so we get
run-time errors. Link explicitly to our libperl.
Also unify 2x -fstack-protector handling as it might not be cygwin only.
tests: small fixes on i148, i105
C: bump to 1.42_70 for ptr_undef handling

More changelog updates
t/issue169.t t/issue203.t TODO fix with next version
todo failing 5.18 threaded tests: 212, 232, 172
todo one more failing 5.18 test: 172 -2
todo more failing 5.18 bc tests
Revert "todo more failing 5.18 bc tests"

This reverts commit 68318ac15de6eb28debaa4154e3b7778c1c63057.
needed a make clean, all those tests pass.
C: fix threaded test 10, store more coresyms properties

test 10 failed by missing fields for *_

store now also non HV entries for *ENV, *SIG,
non-AV entries for *ARGV,
non-AV and non-HV entries for *!
and all for ^H,_,@,^R
module regressions with 1.42_70 1151850

esp. with 5.16
more module regressions with 1.42_70 351d1f3

even before 115185046 fix threaded test 10
update MANIFEST, add 5.19 todos #172,#90
t/issue201.t: cleanup err file
t/issue201.t: add NO_AUTHOR
C: revert parts of the ptr_undef changes, fixes 91,263 missing stashnames

for PVMG entries (mainly magic ISA elements) the pv was not stored.
also optimize such set-magic entries to be static, as they do not grow, the set-magic assigns it.
use 0 instead of ptr_undef for normal PVs, just comppadnames need PL_sv_undef apparently
test: use better temp. output names (for killcccode)
C: more ptr_undef => NULL reversions

NULL is now the default SvPVX, only keep &PL_sv_undef for PVMG
(comppad names)
Also simplify a lot of {.svu_pv=(char*)NULL} to {0} manually