Enable and fix -Wunused-variable warnings.

configure.ac

 dnl Reference: https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html
 dnl ================================================================
 
-AX_CHECK_COMPILE_FLAG([-Wunused-function], [CFLAGS="-Wunused-function $CFLAGS"])
-
+AX_CHECK_COMPILE_FLAG([-Wunused-function],  [CFLAGS="-Wunused-function $CFLAGS"])
 AX_CHECK_COMPILE_FLAG([-Wunused-parameter], [CFLAGS="-Wunused-parameter $CFLAGS"])
+AX_CHECK_COMPILE_FLAG([-Wunused-variable],  [CFLAGS="-Wunused-variable $CFLAGS"])
 
 dnl ================================================================
 dnl Generate output.

src/libespeak-ng/compiledata.c

 static void EndPhonemeTable()
 {
 	int ix;
-	int *pw;
-	int length;
 
 	if (n_phoneme_tabs == 0)
 		return;

src/libespeak-ng/readclause.c

 static int GetC(void)
 {
 	int c1;
-	static int ungot2 = 0;
 
 	if ((c1 = ungot_char) != 0) {
 		ungot_char = 0;

src/libespeak-ng/synthdata.c

 	int length = 0;
 	int rate;
 	unsigned char *p;
-	int *pw;
 
 	espeak_ng_STATUS status;
 	if ((status = ReadPhFile((void **)&phoneme_tab_data, "phontab", NULL, context)) != ENS_OK)
 	unsigned int data;
 	int instn;
 	int instn2;
-	int count;
 	int check_endtype = 0;
 	PHONEME_TAB *ph;
 	PHONEME_LIST *plist_this;

src/libespeak-ng/translate.c

 	int more_suffixes;
 	int confirm_prefix;
 	int spell_word;
-	int stress_bits;
 	int emphasize_allcaps = 0;
 	int wflags;
 	int wmark;

tests/readclause.c

 static int charix_top = 0;
 static int tone2;
 static char voice_change_name[40];
-static int terminator;
 
 static espeak_ng_STATUS
 set_text(const char *text, const char *voicename)