Undeclared dependency Env.pm

[eserte]

Env.pm is a core module, but some distributions (e.g. Fedora) ship a stripped perl installation without it. Probably it's better to add Env to the prereq list to avoid unnecessary test failures:

Can't locate Env.pm in @INC (@INC contains: ... .) at /home/cpansand/.cpan/build/2018111920/FFI-CheckLib-0.23-0iHlUt/blib/lib/FFI/CheckLib.pm line 34.
Compilation failed in require at t/ffi_checklib__os_mswin32.t line 4.
BEGIN failed--compilation aborted at t/ffi_checklib__os_mswin32.t line 4.
t/ffi_checklib__os_mswin32.t .. 
Dubious, test returned 2 (wstat 512, 0x200)
No subtests run