Windows: download busybox-win32 during make win-extras

copy it to echo.exe and printf.exe during make dist
This allows test/spawn.jl to succeed on Windows, since echo and printf
are only included as shell scripts in msysgit (and an echo.bat in
contrib/windows), but libuv cannot spawn shell or bat scripts.

LICENSE.md

@@ -51,3 +51,4 @@ External libraries, if used, include their own licenses:
 - [RMATH](http:https://www.r-project.org/Licenses/)
 - [SUITESPARSE](http:https://www.cise.ufl.edu/research/sparse/SuiteSparse/current/SuiteSparse/)
 - [ZLIB](http:https://zlib.net/zlib_license.html)
+- [BUSYBOX](https://github.com/rmyorston/busybox-w32/blob/master/LICENSE)

Makefile

@@ -271,7 +271,9 @@ ifeq ($(OS), WINNT)
  [ ! -d dist-extras ] || ( cd dist-extras && \
  cp 7z.exe 7z.dll libexpat-1.dll zlib1.dll $(bindir) && \
  mkdir $(DESTDIR)$(prefix)/Git && \
- 7z x PortableGit.7z -o"$(DESTDIR)$(prefix)/Git" )
+ 7z x PortableGit.7z -o"$(DESTDIR)$(prefix)/Git" && \
+ cp busybox.exe $(DESTDIR)$(prefix)/Git/bin/echo.exe && \
+ cp busybox.exe $(DESTDIR)$(prefix)/Git/bin/printf.exe )
  cd $(DESTDIR)$(bindir) && rm -f llvm* llc.exe lli.exe opt.exe LTO.dll bugpoint.exe macho-dump.exe
  $(call spawn,./dist-extras/nsis/makensis.exe) -NOCD -DVersion=$(JULIA_VERSION) -DArch=$(ARCH) -DCommit=$(JULIA_COMMIT) ./contrib/windows/build-installer.nsi
  ./dist-extras/7z a -mx9 "julia-install-$(JULIA_COMMIT)-$(ARCH).7z" julia-installer.exe
@@ -387,10 +389,12 @@ endif
  cd dist-extras && \
  $(JLDOWNLOAD) http:https://downloads.sourceforge.net/sevenzip/7z920_extra.7z && \
  $(JLDOWNLOAD) https://unsis.googlecode.com/files/nsis-2.46.5-Unicode-setup.exe && \
+ $(JLDOWNLOAD) ftp:https://ftp.tigress.co.uk/public/gpl/6.0.0/busybox/busybox.exe && \
  chmod a+x 7z.exe && \
  chmod a+x 7z.dll && \
  $(call spawn,./7z.exe) x -y -onsis nsis-2.46.5-Unicode-setup.exe && \
  chmod a+x ./nsis/makensis.exe && \
+ chmod a+x busybox.exe && \
  7z x -y mingw-libexpat.rpm -so > mingw-libexpat.cpio && \
  7z e -y mingw-libexpat.cpio && \
  7z x -y mingw-zlib.rpm -so > mingw-zlib.cpio && \