fix: uses libc iconv on FreeBSD

[shenlebantongying]

The GNU libiconv has this thing that rename iconv_open to libiconv_open when LIBICONV_PLUG not defined.

The flag LIBICONV_PLUG effectively turns GNU libiconv's header to the base system's iconv header. This might be abuse, but SDL also does this.

https://github.com/libsdl-org/SDL/blob/d6ebbc2fa4abdbe0bd53d0ce8804a492ecb042b9/src/stdlib/SDL_iconv.c#L27-L28

Combined with #1380 should close #1369

Is FreeBSD's libc iconv good enough?

Based on the early design doc of FreeBSD's Iconv, it supports a similar dynamic loading mechanism to GNU libconv so encoding supports should not be a problem.

http://citrus.bsdclub.org/

One can add new locale and encoding without recompiling system libraries by dynamic linking.

[sonarcloud]

Quality Gate passed

Kudos, no new issues were introduced!

0 New issues
0 Security Hotspots
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