As fork(2) is deprecated, its calls must be guarded by
`COMPILER_WARNING_IGNORED(-Wdeprecated-declarations)`.
All usages of fork(2) in process have been alread guarded. A new call
to fork(2) was added in ruby.c with f22c4ff359.
This caused a build failure on Solaris 11.
It may hide a bug to guard big code unnecessarily, so this change
introduces a simple wrapper "rb_fork" whose definition is guarded, and
replaces all calls to fork(2) with the wrapper function.
According to MSVC manual (*1), cl.exe can skip including a header file
when that:
- contains #pragma once, or
- starts with #ifndef, or
- starts with #if ! defined.
GCC has a similar trick (*2), but it acts more stricter (e. g. there
must be _no tokens_ outside of #ifndef...#endif).
Sun C lacked #pragma once for a looong time. Oracle Developer Studio
12.5 finally implemented it, but we cannot assume such recent version.
This changeset modifies header files so that each of them include
strictly one #ifndef...#endif. I believe this is the most portable way
to trigger compiler optimizations. [Bug #16770]
*1: https://docs.microsoft.com/en-us/cpp/preprocessor/once
*2: https://gcc.gnu.org/onlinedocs/cppinternals/Guard-Macros.html
One day, I could not resist the way it was written. I finally started
to make the code clean. This changeset is the beginning of a series of
housekeeping commits. It is a simple refactoring; split internal.h into
files, so that we can divide and concur in the upcoming commits. No
lines of codes are either added or removed, except the obvious file
headers/footers. The generated binary is identical to the one before.
