Use a buildcustomize.pl to set @INC in miniperl when building extensions.

With the build tools now shipped in various subdirectories of cpan/ and dist/
we need to add several paths to @INC when invoking MakeMaker (etc) to build
extensions.

The previous approach of using $ENV{PERL5LIB} was fragile, because:
a: It was hitting the length limit for %ENV variables on VMS
b: It was running the risk of race conditions in a parallel build -
   ExtUtils::Makemaker "knows" to add -I../..lib, which puts lib at the *front*
   of @INC, but if one parallel process happens to copy a module into lib/
   whilst another is searching for it, the second may get a partial read
c: Overwriting $ENV{PERL5LIB} breaks any system where any of the installed
   build tools are actually implemented in Perl, if they are relying on
   $ENV{PERL5LIB} for setup

This approach

a: Doesn't have %ENV length limits
b: Ensures that lib/ is last, so copy targets are always shadowing copy
   sources
c: Only affects miniperl, and doesn't touch $ENV{PERL5LIB}

Approaches that turned out to have fatal flaws:

1: Using $ENV{PERL5OPT} with a module fails because ExtUtils::MakeMaker
   searches for the build perl without setting lib, and treats the error
   caused by a failed -M as "not a valid perl 5 binary"
2: Refactoring ExtUtils::MakeMaker to *not* use -I for lib, and instead rely
   on $ENV{PERL5LIB} [which includes "../../lib"] fails because:
   some extensions have subdirectories, and on these EU::MM correctly uses
   -I../../../lib, where as $ENV{PERL5LIB} only has space for relative paths,
   and only with two levels.

This approach actually takes advantage of ExtUtils::MakeMaker setting an -I
option correct for the depth of directory being built.

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