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
Revert "C: unify empty PVs to ptr_undef"

This reverts commit 6619aad7f9b2f62e9540203f81a4d5cac6d81ac3.
It caused some regressions in module tests, esp. with 5.16, but more
Revert "C: fix threaded test 10, store more coresyms properties"

This reverts commit 1151850466f975b98eb1fb63a6a0405b68bc16c8.
t/issue201.t: cleanup err file