configure.ac: Disable metalink support if an incompatible SSL/TLS specified

tool_metalink only supports cryptography from OpenSSL, GnuTLS, NSS,
The Win32 Crypto library and Apple's Common Crypto library.

If an TLS backend such as mbedTLS or WolfSSL is specified then the
following error is given during compilation along, with a load of
unresolved extern errors:

Can't compile METALINK support without a crypto library.

Reviewed-by: Daniel Stenberg
Closes #5006

configure.ac

@@ -2770,6 +2770,12 @@ if test X"$OPT_LIBMETALINK" != Xno; then
       AC_MSG_NOTICE([libmetalink library defective or too old])
       want_metalink="no"
     ])
+    if test "x$OPENSSL_ENABLED" != "x1" -a "x$USE_WINDOWS_SSPI" != "x1" \
+        -a "x$GNUTLS_ENABLED" != "x1" -a "x$MBEDTLS_ENABLED" != "x1" \
+        -a "x$NSS_ENABLED" != "x1" -a "x$SECURETRANSPORT_ENABLED" != "x1"; then
+      AC_MSG_WARN([metalink support requires a compatible SSL/TLS backend])
+      want_metalink="no"
+    fi
     CPPFLAGS="$clean_CPPFLAGS"
     LDFLAGS="$clean_LDFLAGS"
     LIBS="$clean_LIBS"