Fix build on OpenBSD

[ChihHao-Su]

Tested on OpenBSD 7.3 and Fedora 41

This pull request has a series of modifications, let me explain:

Change the system-detecting logic (configure.ac)

The original configure.ac rejects configuration if the target system is not one of: windows (cygwin/msys), linux, macos. But, actually, this project has basically nothing platform-dependent. Therefore I modified this area of logic: the script will assume the project is able to build on every systems, it just needs to have some special handling for certain platforms.

Get fmt and spdlog via pkg-config instead of specifing -lfmt and -lspdlog flags directly

The libraries might have different filename than we thought, or inside some other directories. For example, on OpenBSD, some libraries are located in /usr/local/lib, for them, additional linker flags or compiler flags are needed.

Give another option for enabling mariadb

So that users can specify clearly whether he wants libmysqlclient or libmariadb.

Remove linker flags link to libstdc++

Target platform may not use libstdc++.

Prevent the including of build-tools/grammar-tools/FlexLexer.h on non-windows platform

build-tools/grammar-tools/FlexLexer.h, this file only works with GNU systems's flex. So just use FlexLexer.h of the system's flex.

Refactor yyin.getline into two overloaded yyinGetline function (libgixpp/GixEsqlLexer.cpp)

In OpenBSD's flex, yyin is a std::istream* instead of a std::istream.

Remove all #include <malloc.h>

This is useless, because there's already #include <stdlib.h>. And this file is not standard C header, doesn't exist on OpenBSD and some systems.

Provide a unified and cross-platform bitswap implementation (runtime/libgixsql/SqlVar.cpp)

To find the bitswap function of every system is troublesome, and you cannot find that on some systems, for example, OpenBSD. Let use a self-implemented one.