Avoid multiple same warnings/errors during poac build

[ken-matsui]

Especially when a header file is included by multiple source files, multithreaded compilations emit the same warnings/errors for the same header file. If there's a way to avoid this (terminate other compilations as soon as one emits errors?), it would be great. We can have alternative solutions for warnings and errors, respectively.

[ken-matsui]

GCC supports -fdiagnostics-format=sarif-stderr, which can be used to merge diagnostics across threads. One possible problem is that we cannot get diagnostics until we finish merging.