build: remove libressl check

Now that BIP66 passed, OpenSSL is no longer directly part of the
consensus. What matters is that DER signatures are correctly parsed, and
secp256k1 crypto is implemented correctly (as well as the other
functions we use from OpenSSL, such as random number generation)

This means that effectively, using LibreSSL is not a larger risk than
using another version of OpenSSL.

Remove the specific check for LibreSSL.

Includes the still-relevant part of bitcoin#6729: make sure CHECK_HEADER is
called using the right CXXFLAGS, not CFLAGS (as AC_LANG is c++).

configure.ac

@@ -700,27 +700,12 @@ else
   fi
 fi
 
-CFLAGS_TEMP="$CFLAGS"
+CXXFLAGS_TEMP="$CXXFLAGS"
 LIBS_TEMP="$LIBS"
-CFLAGS="$CFLAGS $SSL_CFLAGS $CRYPTO_CFLAGS"
+CXXFLAGS="$CXXFLAGS $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"
+CXXFLAGS="$CXXFLAGS_TEMP"
 LIBS="$LIBS_TEMP"
 
 BITCOIN_QT_PATH_PROGS([PROTOC], [protoc],$protoc_bin_path)