configure: Detect (and reject) LibreSSL

Rebased-From: a5a81f7354b3aa3e797d973a7e6840f0e50e6533
Github-Pull: #6244

configure.ac

@@ -677,6 +677,14 @@ else
   fi
 fi
 
+AC_CHECK_LIB([crypto],[RAND_egd],[],[
+  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"