miniruby is used by tool/runruby.rb, so we need to ensure we don't rb_bug
when RUBY_GC_LIBRARY_PATH is set so we can run tests using the make
commands. This commit changes it to warn instead.

On Windows, the error exists on the stack so we should pass an error
buffer from the caller.

Before, if dln_open failed to open RUBY_GC_LIBRARY_PATH, it would segfault
because it would try to raise an error, which cannot happen because the
GC has not been initialized yet.

This commit changes dln_open to return the error that occurred so the
caller can handle the error.

Co-Authored-By: Peter Zhu <peter@peterzhu.ca>

The symbol resolved by dln_symbol will eventually be passed to
extensions. The error handling of dln_sym is also separated into
dln_sym_func because the new call resolving symbols will not raise
LoadError.

a.out format is considered extinct nowadays.

defines.h already has them.  Also __cplusplus can never be defined here.

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