Make POSIX loading a require in bundled version::vpp

Highlighted during core integration testing, lack of POSIX during
bootstrapping core build totally breaks core build.

*sigh*
Fix build after last commit on < 5.10.x
Release engineering for v6.99_14
Fix internal links
Merge pull request #157 from Perl-Toolchain-Gang/fix/pod

Fix internal links
[PATCH] If core, add ccwarnflags and ccstdflags, if available.

Signed-off-by: Chris 'BinGOs' Williams <chris@bingosnet.co.uk>
Release engineering for v6.99_15
Use $ENV{PERL_SRC} to find perl header files too

<@[Tux]> If I use a uninstalled perl where I set $PERL_SRC to something valid, EU::MM is severely broke in sub _perl_header_files {
<@[Tux]> at the point, $self->{PERL_SRC} equals empty
<@[Tux]> I have to pass it as an argument to make it work :(
<@[Tux]>     my $header_dir = $self->{PERL_SRC} || $ENV{PERL_SRC} || $self->catdir($Config{archlibexp}, 'CORE');
<@[Tux]> would be *the* solution IMHO