Build with -Wall -Werror

And fix a couple of related warnings.

Makefile.am

-AM_CPPFLAGS = -I$(top_srcdir)/engine $(SNDFILE_CFLAGS) $(SAMPLERATE_CFLAGS)
+AM_CPPFLAGS = -Wall -Werror -I$(top_srcdir)/engine $(SNDFILE_CFLAGS) $(SAMPLERATE_CFLAGS)
 
 noinst_LTLIBRARIES = libengine.la
 

aux/verify-voice.c

@@ -35,7 +35,7 @@ int main(int argc, char **argv) {
   struct voice voice;
   struct instrument *ins;
   struct layer *layer;
-  float *out_l, *out_r;
+  float *out_l = NULL, *out_r = NULL;
   int total_samples;
   int lead_samples = 100;
   int note_samples = 100000;
@@ -88,8 +88,10 @@ int main(int argc, char **argv) {
   write_sample("out.wav", out_l, out_r, total_samples);
   
  cleanup:
-  free(out_l);
-  free(out_r);
+  if (out_l)
+    free(out_l);
+  if (out_r)
+    free(out_r);
   drumkit_free(&kit);
 
   /*

engine/drumkit.c

@@ -473,7 +473,7 @@ int drumkit_load(struct drumkit *kit, const char *filename, int target_samplerat
   // libxml2 to parse it anyway by setting the proper namespace on all
   // elements.
   root_element = xmlDocGetRootElement(doc);
-  if (!strcmp(root_element->name, "drumkit_info") && root_element->ns == NULL) {
+  if (!strcmp((const char *)root_element->name, "drumkit_info") && root_element->ns == NULL) {
     fprintf(stderr, "spotted old-style (bad) XML\n");
     ns = xmlNewNs(root_element, (xmlChar *)"http://www.hydrogen-music.org/drumkit", (xmlChar *)"");
     fix_bad_xml(root_element, ns);