Add DO NOT EDIT notice to buildcustomize.pl
Do not clobber @INC completely in buildcustomize.pl

buildcustomize.pl (for miniperl) replaces @INC (usually qw(lib .))
with the whole list of build directories we need followed by
qw(lib .).

runperl from test.pl is such that this:

    runperl(switches => [ "-Irun/flib" ], ...)

turns into:

    /path/to/perl.git/miniperl "-I../lib" -Irun/flib

The end result is that -Irun/flib gets stripped out and clobbered, and
switchM.t fails under minitest.
Get switches.t to pass under miniperl
crlf.t: Add skip count
Skip eintr_print.t under miniperl
t/uni/variables.t: Skip $[ test under miniperl
[Merge] Get minitest passing again

One significant change is to the way miniperl gets its @INC set up.
Now buildcustomize.pl replaces lib with its special list, instead of
clobbering the entirety of @INC.  This allows tests to use runperl
with -I, without having to go through hoops to get things to work.