Fix -Wint-conversion warnings.

configure.ac

@@ -304,6 +304,7 @@ dnl Reference: https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html
 dnl ================================================================
 
 AX_CHECK_COMPILE_FLAG([-Wimplicit],              [CFLAGS="-Wimplicit $CFLAGS"])
+AX_CHECK_COMPILE_FLAG([-Wint-conversion],        [CFLAGS="-Wint-conversion $CFLAGS"])
 AX_CHECK_COMPILE_FLAG([-Wmissing-prototypes],    [CFLAGS="-Wmissing-prototypes $CFLAGS"])
 AX_CHECK_COMPILE_FLAG([-Wreturn-type],           [CFLAGS="-Wreturn-type $CFLAGS"])
 AX_CHECK_COMPILE_FLAG([-Wuninitialized],         [CFLAGS="-Wuninitialized $CFLAGS"])

src/libespeak-ng/synthesize.c

@@ -136,7 +136,7 @@ static void DoAmplitude(int amp, unsigned char *amp_env)
 static void DoPhonemeAlignment(char* pho, int type)
 {
 	wcmdq[wcmdq_tail][0] = WCMD_PHONEME_ALIGNMENT;
-	wcmdq[wcmdq_tail][1] = pho;
+	wcmdq[wcmdq_tail][1] = (intptr_t)pho;
 	wcmdq[wcmdq_tail][2] = type;
 	WcmdqInc();
 }