How about creating a default property 'false' with a condition to assign true if the condition passes?
<PropertyGroup>
<MatchesInclude>false</MatchesInclude>
<MatchesInclude Condition="'$(IncludeInBuild)'=='' OR
'$([System.Text.RegularExpressions.Regex]::IsMatch($(MSBuildProjectFullPath),
$(IncludeInBuild)'=='True'">true</MatchesInclude>
</PropertyGroup>