This option instructs the compiler to use a PCH file if it exists, and to create a PCH file otherwise.
When the option
--pch is specified, the compiler
searches for a PCH file with the name
the name of the primary source file. The compiler uses the PCH file
it exists, and creates a PCH file named
the same directory as the primary source file otherwise.
This option has no effect if you include either the option
the same command line.