Add some compiler warnings and corresponding fixes.

Most of the current problems were (mostly) harmless things like
missing declarations, but there was at least one real error, (reversed
argument order for yyerrror).
diff --git a/glcpp.c b/glcpp.c
index 90a0e89..eefac74 100644
--- a/glcpp.c
+++ b/glcpp.c
@@ -21,6 +21,8 @@
  * DEALINGS IN THE SOFTWARE.
  */
 
+#include "glcpp.h"
+
 int
 main (void)
 {