Hook memmem compat code into build.

This fixes builds on platforms that don't have it (at least old DragonFly,
probably others).

configure.ac

@@ -1751,6 +1751,7 @@ AC_CHECK_FUNCS([ \
 	llabs \
 	login_getcapbool \
 	md5_crypt \
+	memmem \
 	memmove \
 	memset_s \
 	mkdtemp \

openbsd-compat/Makefile.in

@@ -34,6 +34,7 @@ OPENBSD=base64.o \
 	inet_ntoa.o \
 	inet_ntop.o \
 	md5.o \
+	memmem.o \
 	mktemp.o \
 	pwcache.o \
 	readpassphrase.o \

openbsd-compat/memmem.c

@@ -27,6 +27,10 @@
  * SUCH DAMAGE.
  */
 
+#include "includes.h"
+
+#ifndef HAVE_MEMMEM
+
 #include <string.h>
 
 /*
@@ -62,3 +66,4 @@ memmem(const void *l, size_t l_len, const void *s, size_t s_len)
 	return NULL;
 }
 DEF_WEAK(memmem);
+#endif /* HAVE_MEMMEM */

openbsd-compat/openbsd-compat.h

@@ -73,6 +73,10 @@ int getpagesize(void);
 char *getcwd(char *pt, size_t size);
 #endif
 
+#ifdef HAVE_MEMMEM
+void *memmem(const void *, size_t, const void *, size_t);
+#endif
+
 #ifndef HAVE_REALLOCARRAY
 void *reallocarray(void *, size_t, size_t);
 #endif