--pch command-line option enables automatic
PCH file processing. This means that the compiler automatically
looks for a qualifying PCH file, and reads it if found. Otherwise,
the compiler creates one for use on a subsequent compilation.
When the compiler creates a PCH file, it takes the name of
the primary source file and replaces the suffix with
The PCH file is created in the directory of the primary source file
--pch_dir option is specified.