Include the configuration condition for ExternalWarningLevel.

[LoneBoco]

What does this PR do?

Fixes a bug with ExternalWarningLevel where the entry would be duplicated for every configuration you had.

Before:

<ClCompile Include="file.cpp">
  <ExternalWarningLevel>Level3</ExternalWarningLevel>
  <ExternalWarningLevel>Level3</ExternalWarningLevel>
  <ExternalWarningLevel>Level3</ExternalWarningLevel>
  <ExternalWarningLevel>Level3</ExternalWarningLevel>
</ClCompile>

After:

<ClCompile Include="file.cpp">
  <ExternalWarningLevel>Level3</ExternalWarningLevel>
</ClCompile>

How does this PR change Premake's behavior?

No breaking changes.

Anything else we should know?

The bug was introduced with changeset 065b3ac.

Did you check all the boxes?

• Focus on a single fix or feature; remove any unrelated formatting or code changes
• Add unit tests showing fix or feature works; all tests pass
• Mention any related issues (put closes #XXXX in comment to auto-close issue when PR is merged)
• Follow our coding conventions
• Minimize the number of commits
• Align documentation to your changes

You can now support Premake on our OpenCollective. Your contributions help us spend more time responding to requests like these!

[Jarod42]

You checked "Add Unit tests", but don't provide tests to show old bug.

[LoneBoco]

Okay, I fixed all instances of file level handling of externalwarnings and externalanglebrackets, including adding tests for them.

[LoneBoco]

I also prevented it from exporting on the file level if it wasn't declared.