Fix iconv in eglib, remove GNU libiconv-ism

iconv in eglib was broken due to a broken define. Also fix linking
of iconv when iconv isn't in libc.

Also remove the usage of a GNU libcharset specific function and rely
on a standard POSIX function that outputs the same value on every
modern Unixoid I've tried it on. (Ubuntu/macOS/i/FreeBSD)

This comment explains a lot of things for the latter:

```c
/*
 * This function used used to use the "locale_charset" call in
 * libiconv's libcharset library. However, this isn't always
 * available on some systems, including ones with GNU libc. So,
 * instead use a function that's a standard part of POSIX2008.
 *
 * nl_langinfo is in POSIX2008 and should be on any sane modern
 * Unix. With a UTF-8 locale, it should return "UTF-8" - this
 * has been verified with Ubuntu 18.04, FreeBSD 11, and i 7.3.
 *
 * The motivation for using locale_charset was likely due to
 * the cruftiness of Unices back in ~2001; where you had to
 * manually query environment variables, and the values were
 * inconsistent between each other. Nowadays, if Linux, macOS,
 * AIX/PASE, and FreeBSD can all return the same values for a
 * UTF-8 locale, we can just use the value directly.
 *
 * It should be noted that by default, this function will give
 * values for the "C" locale, unless `setlocale (LC_ALL, "")`
 * is ran to init locales - driver.c in mini does this for us.
 */
```

1 files changed, 4 insertions, 2 deletions

diff --git a/configure.ac b/configure.ac
index 6988629a8ee..0cd0ec940b6 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1904,6 +1904,7 @@ if test x$host_win32 = xno; then
 	AC_CHECK_FUNCS(getrlimit)
 	AC_CHECK_FUNCS(prctl)
 	AC_CHECK_FUNCS(arc4random)
+	AC_CHECK_FUNCS(nl_langinfo)
 
 	AC_CHECK_FUNCS(sched_getaffinity)
 	AC_CHECK_FUNCS(sched_setaffinity)
@@ -4593,8 +4594,9 @@ if test $found_export_dynamic = no; then
 	LDFLAGS="${old_ldflags}"
 fi
 
-AC_CHECK_HEADERS(getopt.h sys/select.h sys/time.h sys/wait.h pwd.h iconv.h localcharset.h sys/types.h sys/resource.h)
-AC_CHECK_LIB([iconv], [locale_charset],[],[AC_CHECK_LIB([charset], [locale_charset],[LIBS+=" -liconv -lcharset"])])
+AC_CHECK_HEADERS(getopt.h sys/select.h sys/time.h sys/wait.h pwd.h iconv.h sys/types.h sys/resource.h)
+dnl giconv.c will check on HAVE_ICONV_H but we need this for link time
+AC_CHECK_LIB(iconv, iconv_open)
 AC_CHECK_HEADER(alloca.h, [HAVE_ALLOCA_H=1], [HAVE_ALLOCA_H=0])
 AC_SUBST(HAVE_ALLOCA_H)