diff --git a/.gitignore b/.gitignore
index 47826973..36d797e3 100644
--- a/.gitignore
+++ b/.gitignore
@@ -66,11 +66,13 @@ depcomp
 install-sh
 libtool
 ltmain.sh
-m4/
 missing
 stamp-h1
 .dirstamp
 
+m4/*
+!m4/ax_check_compile_flag.m4
+
 # libraries
 
 docs/speak_lib.h
diff --git a/configure.ac b/configure.ac
index 691e31d1..f51c54ee 100644
--- a/configure.ac
+++ b/configure.ac
@@ -269,6 +269,14 @@ AM_CONDITIONAL(HAVE_RU_EXTENDED_DICTIONARY,  [test x"$have_extdict_ru"  = xyes])
 AM_CONDITIONAL(HAVE_ZH_EXTENDED_DICTIONARY,  [test x"$have_extdict_zh"  = xyes])
 AM_CONDITIONAL(HAVE_ZHY_EXTENDED_DICTIONARY, [test x"$have_extdict_zhy" = xyes])
 
+dnl ================================================================
+dnl Compiler warnings.
+dnl
+dnl Reference: https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html
+dnl ================================================================
+
+AX_CHECK_COMPILE_FLAG([-Wunused-function], [CFLAGS="-Wunused-function $CFLAGS"])
+
 dnl ================================================================
 dnl Generate output.
 dnl ================================================================
diff --git a/m4/ax_check_compile_flag.m4 b/m4/ax_check_compile_flag.m4
new file mode 100644
index 00000000..80c79114
--- /dev/null
+++ b/m4/ax_check_compile_flag.m4
@@ -0,0 +1,75 @@
+# ===========================================================================
+#  https://www.gnu.org/software/autoconf-archive/ax_check_compile_flag.html
+# ===========================================================================
+#
+# SYNOPSIS
+#
+#   AX_CHECK_COMPILE_FLAG(FLAG, [ACTION-SUCCESS], [ACTION-FAILURE], [EXTRA-FLAGS], [INPUT])
+#
+# DESCRIPTION
+#
+#   Check whether the given FLAG works with the current language's compiler
+#   or gives an error.  (Warnings, however, are ignored)
+#
+#   ACTION-SUCCESS/ACTION-FAILURE are shell commands to execute on
+#   success/failure.
+#
+#   If EXTRA-FLAGS is defined, it is added to the current language's default
+#   flags (e.g. CFLAGS) when the check is done.  The check is thus made with
+#   the flags: "CFLAGS EXTRA-FLAGS FLAG".  This can for example be used to
+#   force the compiler to issue an error when a bad flag is given.
+#
+#   INPUT gives an alternative input source to AC_COMPILE_IFELSE.
+#
+#   NOTE: Implementation based on AX_CFLAGS_GCC_OPTION. Please keep this
+#   macro in sync with AX_CHECK_{PREPROC,LINK}_FLAG.
+#
+# LICENSE
+#
+#   Copyright (c) 2008 Guido U. Draheim <guidod@gmx.de>
+#   Copyright (c) 2011 Maarten Bosmans <mkbosmans@gmail.com>
+#
+#   This program is free software: you can redistribute it and/or modify it
+#   under the terms of the GNU General Public License as published by the
+#   Free Software Foundation, either version 3 of the License, or (at your
+#   option) any later version.
+#
+#   This program is distributed in the hope that it will be useful, but
+#   WITHOUT ANY WARRANTY; without even the implied warranty of
+#   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General
+#   Public License for more details.
+#
+#   You should have received a copy of the GNU General Public License along
+#   with this program. If not, see <https://www.gnu.org/licenses/>.
+#
+#   As a special exception, the respective Autoconf Macro's copyright owner
+#   gives unlimited permission to copy, distribute and modify the configure
+#   scripts that are the output of Autoconf when processing the Macro. You
+#   need not follow the terms of the GNU General Public License when using
+#   or distributing such scripts, even though portions of the text of the
+#   Macro appear in them. The GNU General Public License (GPL) does govern
+#   all other use of the material that constitutes the Autoconf Macro.
+#
+#   This special exception to the GPL applies to versions of the Autoconf
+#   Macro released by the Autoconf Archive. When you make and distribute a
+#   modified version of the Autoconf Macro, you may extend this special
+#   exception to the GPL to apply to your modified version as well.
+
+#serial 5
+
+AC_DEFUN([AX_CHECK_COMPILE_FLAG],
+[AC_PREREQ(2.64)dnl for _AC_LANG_PREFIX and AS_VAR_IF
+AS_VAR_PUSHDEF([CACHEVAR],[ax_cv_check_[]_AC_LANG_ABBREV[]flags_$4_$1])dnl
+AC_CACHE_CHECK([whether _AC_LANG compiler accepts $1], CACHEVAR, [
+  ax_check_save_flags=$[]_AC_LANG_PREFIX[]FLAGS
+  _AC_LANG_PREFIX[]FLAGS="$[]_AC_LANG_PREFIX[]FLAGS $4 $1"
+  AC_COMPILE_IFELSE([m4_default([$5],[AC_LANG_PROGRAM()])],
+    [AS_VAR_SET(CACHEVAR,[yes])],
+    [AS_VAR_SET(CACHEVAR,[no])])
+  _AC_LANG_PREFIX[]FLAGS=$ax_check_save_flags])
+AS_VAR_IF(CACHEVAR,yes,
+  [m4_default([$2], :)],
+  [m4_default([$3], :)])
+AS_VAR_POPDEF([CACHEVAR])dnl
+])dnl AX_CHECK_COMPILE_FLAGS
+
diff --git a/src/libespeak-ng/voices.c b/src/libespeak-ng/voices.c
index f657af08..c7a1a75e 100644
--- a/src/libespeak-ng/voices.c
+++ b/src/libespeak-ng/voices.c
@@ -474,18 +474,6 @@ static int Read8Numbers(char *data_in, int *data)
 	              &data[0], &data[1], &data[2], &data[3], &data[4], &data[5], &data[6], &data[7]);
 }
 
-static unsigned int StringToWord2(const char *string)
-{
-	// Convert a language name string to a word such as L('e','n')
-	int ix;
-	int c;
-	unsigned int value = 0;
-
-	for (ix = 0; (ix < 4) && ((c = string[ix]) != 0); ix++)
-		value = (value << 8) | (c & 0xff);
-	return value;
-}
-
 voice_t *LoadVoice(const char *vname, int control)
 {
 	// control, bit 0  1= no_default