Merge pull request dotnet/corefx#36387 from ericstj/sourceBuildFixes
authorViktor Hofer <viktor.hofer@microsoft.com>
Wed, 27 Mar 2019 15:04:36 +0000 (16:04 +0100)
committerGitHub <noreply@github.com>
Wed, 27 Mar 2019 15:04:36 +0000 (16:04 +0100)
Make some packages optional for source build

Commit migrated from https://github.com/dotnet/corefx/commit/04eba1a41579b3f9d9dbfdc72a96977f5897ff9d


Trivial merge