configure: only add OpenSSL paths if they are defined

Add paths for OpenSSL compiling and linking only if they have been
defined.  If they haven't been defined, we'll assume that the paths are
already available to the toolchain.

Closes #6730

configure.ac

@@ -1834,8 +1834,10 @@ if test -z "$ssl_backends" -o "x$OPT_SSL" != xno &&
      HAVECRYPTO="yes"
      LIBS="-lcrypto $LIBS"
      ],[
-     LDFLAGS="$CLEANLDFLAGS -L$LIB_OPENSSL"
-     if test "$PKGCONFIG" = "no" ; then
+     if test -n "$LIB_OPENSSL" ; then
+       LDFLAGS="$CLEANLDFLAGS -L$LIB_OPENSSL"
+     fi
+     if test "$PKGCONFIG" = "no" -a -n "$PREFIX_OPENSSL" ; then
        # only set this if pkg-config wasn't used
        CPPFLAGS="$CLEANCPPFLAGS -I$PREFIX_OPENSSL/include/openssl -I$PREFIX_OPENSSL/include"
      fi