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

apps/Makefile.am.tpl

@@ -10,6 +10,6 @@ openssl_LDADD += $(top_builddir)/crypto/libcrypto.la
 openssl_SOURCES =
 noinst_HEADERS =
 
-if NO_STRTONUM
+if !HAVE_STRTONUM
 openssl_SOURCES += strtonum.c
 endif