configure.ac: define ICONV_OMITS_BOM if necessary

From commit 79444c9, ("utf8: handle systems that don't write BOM for
UTF-16", 2019-02-12), we're supporting those systems with iconv that
omits BOM with:

    make ICONV_OMITS_BOM=Yes

However, configure script wasn't taught to detect those systems.

Teach configure to do so.

Signed-off-by: Doan Tran Cong Danh <congdanhqx@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

configure.ac

@@ -844,12 +844,61 @@ AC_MSG_CHECKING([for old iconv()])
 AC_COMPILE_IFELSE([OLDICONVTEST_SRC],
 	[AC_MSG_RESULT([no])],
 	[AC_MSG_RESULT([yes])
+	AC_DEFINE(HAVE_OLD_ICONV, 1)
 	OLD_ICONV=UnfortunatelyYes])
 
 GIT_UNSTASH_FLAGS($ICONVDIR)
 
 GIT_CONF_SUBST([OLD_ICONV])
 
+#
+# Define ICONV_OMITS_BOM if you are on a system which
+# iconv omits bom for utf-{16,32}
+if test -z "$NO_ICONV"; then
+AC_CACHE_CHECK([whether iconv omits bom for utf-16 and utf-32],
+ [ac_cv_iconv_omits_bom],
+[
+old_LIBS="$LIBS"
+if test -n "$NEEDS_LIBICONV"; then
+	LIBS="$LIBS -liconv"
+fi
+
+AC_RUN_IFELSE(
+	[AC_LANG_PROGRAM([AC_INCLUDES_DEFAULT
+	#include <iconv.h>
+	#ifdef HAVE_OLD_ICONV
+	typedef const char *iconv_ibp;
+	#else
+	typedef char *iconv_ibp;
+	#endif
+	],
+	[[
+	int v;
+	iconv_t conv;
+	char in[] = "a"; iconv_ibp pin = in;
+	char out[20] = ""; char *pout = out;
+	size_t isz = sizeof in;
+	size_t osz = sizeof out;
+
+	conv = iconv_open("UTF-16", "UTF-8");
+	iconv(conv, &pin, &isz, &pout, &osz);
+	iconv_close(conv);
+	v = (unsigned char)(out[0]) + (unsigned char)(out[1]);
+	return v != 0xfe + 0xff;
+	]])],
+	[ac_cv_iconv_omits_bom=no],
+	[ac_cv_iconv_omits_bom=yes])
+
+LIBS="$old_LIBS"
+])
+if test "x$ac_cv_iconv_omits_bom" = xyes; then
+	ICONV_OMITS_BOM=Yes
+else
+	ICONV_OMITS_BOM=
+fi
+GIT_CONF_SUBST([ICONV_OMITS_BOM])
+fi
+
 ## Checks for typedefs, structures, and compiler characteristics.
 AC_MSG_NOTICE([CHECKS for typedefs, structures, and compiler characteristics])
 #