Improve and simplify function and header detection logic.

Simplify autoconf checks by using AC_CHECK_FUNCS/HEADERS.
Clarify some ambiguous dependencies around strnlen/strndup.
Unconditionally enable pidwraptest for all arc4random implementations.
Remove HAVE_VASPRINTF conditional, since asprintf requires vasprintf.

ok @doug

include/string.h

@@ -12,34 +12,35 @@
 #include <strings.h>
 #endif
 
-#ifdef NO_STRLCPY
+#ifndef HAVE_STRLCPY
 size_t strlcpy(char *dst, const char *src, size_t siz);
 #endif
 
-#ifdef NO_STRLCAT
+#ifndef HAVE_STRLCAT
 size_t strlcat(char *dst, const char *src, size_t siz);
 #endif
 
-#ifdef NO_STRNDUP
+#ifndef HAVE_STRNDUP
 char * strndup(const char *str, size_t maxlen);
-#ifdef NO_STRNLEN
+/* the only user of strnlen is strndup, so only build it if needed */
+#ifndef HAVE_STRNLEN
 size_t strnlen(const char *str, size_t maxlen);
 #endif
 #endif
 
-#ifdef NO_EXPLICIT_BZERO
+#ifndef HAVE_EXPLICIT_BZERO
 void explicit_bzero(void *, size_t);
 #endif
 
-#ifdef NO_TIMINGSAFE_BCMP
+#ifndef HAVE_TIMINGSAFE_BCMP
 int timingsafe_bcmp(const void *b1, const void *b2, size_t n);
 #endif
 
-#ifdef NO_TIMINGSAFE_MEMCMP
+#ifndef HAVE_TIMINGSAFE_MEMCMP
 int timingsafe_memcmp(const void *b1, const void *b2, size_t len);
 #endif
 
-#ifdef NO_MEMMEM
+#ifndef HAVE_MEMMEM
 void * memmem(const void *big, size_t big_len, const void *little,
 	size_t little_len);
 #endif