[libarchive] fix build with VS2017

[codicodi]

Libarchive uses check_function_exists module for feature checks. This macro is completly broken when it comes to compiler built-ins and inline functions (see here and here). As in VS2017 wcslen is also a built-in, libarchive fails to detect it and defines its own version - which ends with "built-in function redefinition" error.
This PR provides a patch that skips all problematic checks.

[alexkaratarakis]

Does it build for you? I am getting these errors:

     4>archive_string.obj : error LNK2019: unresolved external symbol __imp__libiconv_open referenced in function _create_sconv_object [E:\workspace\vcpkgspace\buildtrees\libarchive\x86-windows-rel\libarchive\archive.vcxproj]
     4>archive_string.obj : error LNK2019: unresolved external symbol __imp__libiconv referenced in function _iconv_strncat_in_locale [E:\workspace\vcpkgspace\buildtrees\libarchive\x86-windows-rel\libarchive\archive.vcxproj]
     4>archive_string.obj : error LNK2019: unresolved external symbol __imp__libiconv_close referenced in function _archive_string_conversion_free 

Note that I build the whole chain with VS2017 so there might be something with libiconv that we need to check.

[codicodi]

Yes, I successfully built whole chain for x86-windows and x64-windows with VS2017.

[ras0219-msft]

I was able to repro the same issues that @alexkaratarakis hit above, so this may have been introduced in newer versions of the 2017 preview. We should look into investigating further.

However, after looking at the code that actually uses libiconv, it appears to provide extremely limited functionality on Windows due to the existence of the Win32 APIs (which libarchive does use in preference to libiconv). Disabling libiconv did fix the issues, so I've added that during the merge.

Thanks for digging in and figuring out that wcslen issue!