Check if memmem is declared in system headers.

If the system (or one of the dependencies) implements memmem but does
not define the header, we would not declare it either resulting in
compiler warnings.  Check for declaration explicitly.  bz#3102.

configure.ac

@@ -1873,7 +1873,7 @@ AC_CHECK_FUNCS([ \
 	warn \
 ])
 
-AC_CHECK_DECLS([bzero])
+AC_CHECK_DECLS([bzero, memmem])
 
 dnl Wide character support.
 AC_CHECK_FUNCS([mblen mbtowc nl_langinfo wcwidth])

openbsd-compat/openbsd-compat.h

@@ -74,7 +74,7 @@ int getpagesize(void);
 char *getcwd(char *pt, size_t size);
 #endif
 
-#ifndef HAVE_MEMMEM
+#if defined(HAVE_DECL_MEMMEM) && HAVE_DECL_MEMMEM == 0
 void *memmem(const void *, size_t, const void *, size_t);
 #endif