build: fix libressl detection

Checking libcrypto for a function after we've already found a (possibly
different) libcrypto is not what we want to do here.

pkg-config might've found a cross lib while AC_CHECK_LIB may find a different
or native one.

Run a link-test against the lib that's already been found instead.

configure.ac

@@ -692,6 +692,21 @@ LIBS_TEMP="$LIBS"
 CFLAGS="$CFLAGS $SSL_CFLAGS $CRYPTO_CFLAGS"
 LIBS="$LIBS $SSL_LIBS $CRYPTO_LIBS"
 AC_CHECK_HEADER([openssl/ec.h],, AC_MSG_ERROR(OpenSSL ec header missing),)
+
+AC_MSG_CHECKING(for a supported OpenSSL version)
+AC_LINK_IFELSE([AC_LANG_PROGRAM([[
+  #include <openssl/rand.h>
+  ]],
+  [[RAND_egd(NULL);]])],
+  [AC_MSG_RESULT(yes)],
+  [
+    AC_ARG_WITH([libressl],
+    [AS_HELP_STRING([--with-libressl],[Build with system LibreSSL (default is no; DANGEROUS; NOT SUPPORTED)])],
+    [AC_MSG_WARN([Detected LibreSSL: This is NOT supported, and may break consensus compatibility!])],
+    [AC_MSG_ERROR([Detected LibreSSL: This is NOT supported, and may break consensus compatibility!])]
+    )]
+)
+
 CFLAGS="$CFLAGS_TEMP"
 LIBS="$LIBS_TEMP"