Uses a PCH file if it exists, creates a PCH file otherwise.
When the option
--pch is specified, the compiler searches for a PCH file
with the name filename.pch, where
filename.* is the name of the primary source
file. The compiler uses the PCH file filename.pch if
it exists, and creates a PCH file named filename.pch
in the same directory as the primary source file otherwise.
This option has no effect if you include either the option
--use_pch=filename or the option
--create_pch=filename on the same command line.