libHttpClient.GDK.vcxproj: PDB conflict (C1041) when built with different LibHttpClientIncludeSuffix via AdditionalProperties

[jasonsandlin]

When libHttpClient.GDK.vcxproj is built from a solution that triggers both the default build (LibHttpClientIncludeSuffix=true) and a ProjectReference build with AdditionalProperties setting LibHttpClientIncludeSuffix=false, MSBuild treats these as separate builds but both use the same IntDir ($(HCIntRoot)$(Platform)$(Configuration)\libHttpClient.GDK). Concurrent CL.EXE instances from each build collide on vc143.pdb, producing:

error C1041: cannot open program database '...\Int\x64\Release\libHttpClient.GDK\vc143.pdb';
if multiple CL.EXE write to the same .PDB file, please use /FS

Fix: Two changes:

1. Separate IntDir when LibHttpClientIncludeSuffix=false so each build variant has its own intermediate directory
2. Add /FS to ClCompile options as a safety net for concurrent PDB writes

[tculotta]