build-sys: change all HAVE_DECL_ macros to HAVE_

This is a legacy of autotools, where one detection routine used a different
prefix then the others.

$ git grep -e HAVE_DECL_ -l|xargs sed -i s/HAVE_DECL_/HAVE_/g

src/basic/string-util.c

@@ -826,7 +826,7 @@ int free_and_strdup(char **p, const char *s) {
         return 1;
 }
 
-#if !HAVE_DECL_EXPLICIT_BZERO
+#if !HAVE_EXPLICIT_BZERO
 /*
  * Pointer to memset is volatile so that compiler must de-reference
  * the pointer and can't assume that it points to any function in