code cleanup: change global int current_phoneme table to static

It's only used in SpeakNextClause() and related to SelectPhonemeTable().

Another solution void be to keep SelectPhonemeTable() as a void function
and do a current_phoneme_table = voice->phoneme_tab_ix after the call to
SelectPhonemeTable().

src/libespeak-ng/phoneme.h

 // Several phoneme tables may be loaded into memory. phoneme_tab points to
 // one for the current voice
 extern int n_phoneme_tab;
-extern int current_phoneme_table;
 extern PHONEME_TAB *phoneme_tab[N_PHONEME_TAB];
 extern unsigned char phoneme_tab_flags[N_PHONEME_TAB];  // bit 0: not inherited
 

src/libespeak-ng/synthdata.c

 
 // copy the current phoneme table into here
 int n_phoneme_tab;
-int current_phoneme_table;
 PHONEME_TAB *phoneme_tab[N_PHONEME_TAB];
 unsigned char phoneme_tab_flags[N_PHONEME_TAB];   // bit 0: not inherited
 
 	}
 }
 
-void SelectPhonemeTable(int number)
+int SelectPhonemeTable(int number)
 {
 	n_phoneme_tab = 0;
 	SetUpPhonemeTable(number, false); // recursively for included phoneme tables
 	n_phoneme_tab++;
-	current_phoneme_table = number;
+	return number;
 }
 
 int LookupPhonemeTable(const char *name)

src/libespeak-ng/synthdata.h

 
 int NumInstnWords(unsigned short *prog);
 int PhonemeCode(unsigned int mnem);
-void SelectPhonemeTable(int number);
+int SelectPhonemeTable(int number);
 int  SelectPhonemeTableName(const char *name);
 
 #ifdef __cplusplus

src/libespeak-ng/synthesize.c

 	return 0; // finished the phoneme list
 }
 
+static int current_phoneme_table;
 int SpeakNextClause(int control)
 {
 	// Speak text from memory (text_in)
 	}
 
 	if (current_phoneme_table != voice->phoneme_tab_ix)
-		SelectPhonemeTable(voice->phoneme_tab_ix);
+		current_phoneme_table = SelectPhonemeTable(voice->phoneme_tab_ix);
 
 	// read the next clause from the input text file, translate it, and generate
 	// entries in the wavegen command queue