remove explicit paths, and gnutar assumption
move tar check to Make.inc and update tar commands
test for compatible tar, error if not found
Skip unicode tests if `iconv --version` fails
Add instructions for windows users
Extract OPENBLAS_BUILD_OPTS modification for separate PR
Merge pull request #5567 from JuliaLang/sf/qbit_tarfix

Merge testing for compatible tar executables
Map amd64 -> x86_64 in Make.inc
Fix search for bsdtar

Using `tar --help` on `bsdtar` doesn't show a `--strip-components` option like the GNU version (though it does show in the man page. Instead, explicitly check either for `strip-components` or `bsdtar`, which has supported strip-components for the same timeframe as GNU tar, with full support ~2006 for both.
Merge pull request #5569 from scw/patch-1

Fix search for bsdtar
Iteration over ENV (singleton of EnvHash Dict) returned a SubString
instead of a ByteString.  E.g. isequal(ENV,ENV) threw an error.

Now fixed.
Merge pull request #5544 from JuliaLang/sf/iconvskip

Skip unicode tests if `iconv --version` fails
Fix Issue #5581
Merge pull request #5580 from mauro3/ENVgetBug

Iteration over ENV returns wrong type
Merge pull request #5583 from jakebolewski/jb/fix5581

Fix Issue #5581
base/pkg/git.jl: don't shell out to find .git dir [fixes #4888]
Fix #5586
sharedarray - exported data, indexpids, subtype changes