Jessica Hamilton authored Jul 28, 2021 and Brad King committed Jul 29, 2021

The behavior was added by commit 7ebc1cb2 (Haiku: Several fixes to
platform module, 2013-10-05, v3.0.0-rc1~541^2~3), but was not explained
in its commit message.  No other platform modules do this.

The include-once behavior results in incorrect variables set in some
larger projects, e.g. .NET Core, particularly when cross-compiling,
`CMAKE_DL_LIBS` being one of these.  Remove it.