lldpd currently depends on a C++ compiler to configure properly, but
the package doesn't select that option, so builds fail if
BR2_TOOLCHAIN_BUILDROOT_CXX is not selected with following errors:

  checking how to run the C++ preprocessor... /lib/cpp
  configure: error: in `/home/dkc/src/buildroot/build-zii/build/lldpd-0.9.4':
  configure: error: C++ preprocessor "/lib/cpp" fails sanity check

This package actually builds fine without C++, so drop this check in
configure.ac. Attached patch has already been accepted upstream [1].

[1] https://github.com/vincentbernat/lldpd/pull/261



[Peter: adjust autoreconf comment]
Signed-off-by: Damien Riegel <damien.riegel@savoirfairelinux.com>
Reviewed-by: Julien Floret <julien.floret@6wind.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

(cherry picked from commit 18c9cda6e4b94aa9426a1355d05b000f69453c6b)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>