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.h

@@ -189,7 +189,7 @@ static inline void *memmem_safe(const void *haystack, size_t haystacklen, const
         return memmem(haystack, haystacklen, needle, needlelen);
 }
 
-#if !HAVE_DECL_EXPLICIT_BZERO
+#if !HAVE_EXPLICIT_BZERO
 void explicit_bzero(void *p, size_t l);
 #endif