From commit 79444c9294, ("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, typing the flag all the time is cumbersome and error-prone.

Add a checking into configure script to detect this flag automatically.

Signed-off-by: Doan Tran Cong Danh <redacted>
---

Notes:
    We deliberately fail for ac_cv_iconv_omits_bom on cross-compiling,
    in order to ask builder provide the value for the target.
    
    We're relied on this technik for ac_cv_fread_reads_directories and
    ac_cv_snprintf_returns_bogus.
    
    Adding one more failure for configuring on cross-compiling
    is not going to be a burden for distro.

 configure.ac | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/configure.ac b/configure.ac
index a43b476402..790b53bbdc 100644
--- a/configure.ac
+++ b/configure.ac

@@ -690,6 +690,28 @@ fi
 
 fi
 
+#
+# Define ICONV_OMITS_BOM if you are on a system which
+# iconv omits bom for utf-{16,32}
+if test -z "$NO_ICONV"; then
+AC_CACHE_CHECK([whether iconv omits bom for utf-16 and utf-32],
+ [ac_cv_iconv_omits_bom],
+[
+if test "x$cross_compiling" = xyes; then
+	AC_MSG_FAILURE([please provide ac_cv_iconv_omits_bom])
+elif test `printf a | iconv -f utf-8 -t utf-16 | wc -c` = 2; then
+	ac_cv_iconv_omits_bom=yes
+else
+	ac_cv_iconv_omits_bom=no
+fi
+])
+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
 #
 # Define NO_DEFLATE_BOUND if deflateBound is missing from zlib.
 

-- 
2.24.0.rc1.3.g89530838a3.dirty