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/stdio.h

@@ -3,7 +3,7 @@
 #ifndef LIBCRYPTOCOMPAT_STDIO_H
 #define LIBCRYPTOCOMPAT_STDIO_H
 
-#ifdef NO_ASPRINTF
+#ifndef HAVE_ASPRINTF
 #include <stdarg.h>
 int vasprintf(char **str, const char *fmt, va_list ap);
 int asprintf(char **str, const char *fmt, ...);