t/lib/MBTest.pm: When cross-compiling, check the compiler even if PERL_CORE=1

This is because the target system might not have a compiler at all,
or might have a compiler/ccflags combination that doesn't mesh well,
so we can't assume anything.
M::B::Unix::_detildefy: Protect against platforms without getpw{nam,uid}

Unfortunately there is no %Config key to check, so we have to
eval {} the calls; We also make the getpwuid case default to glob("~").
Use guess_license_from_meta_key if available

This is necessary for the artistic licenses for example, because their meta
names don't match their class names.
Prepare for release 0.4205
Bumped to version 0.4206
Make the meta tests more robust to CPAN::Meta changes

As done for ExtUtils::MakeMaker v6.91_01
Enable release and author tests during disttest
update the minimum perl version required

On 5.6.2 I got:

t/manifypods_with_utf8.t ....... Undefined subroutine utf8::encode called at t/manifypods_with_utf8.t line 35.

So the stated prereq of 5.6.1 isn't actually true anymore.