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

@@ -680,19 +680,26 @@ else
   fi
 fi
 
-AC_CHECK_LIB([crypto],[RAND_egd],[],[
+CFLAGS_TEMP="$CFLAGS"
+LIBS_TEMP="$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_TEMP="$CFLAGS"
-LIBS_TEMP="$LIBS"
-CFLAGS="$CFLAGS $SSL_CFLAGS $CRYPTO_CFLAGS"
-LIBS="$LIBS $SSL_LIBS $CRYPTO_LIBS"
-AC_CHECK_HEADER([openssl/ec.h],, AC_MSG_ERROR(OpenSSL ec header missing),)
 CFLAGS="$CFLAGS_TEMP"
 LIBS="$LIBS_TEMP"