package/ola: fix autoreconf issue for host-ola

CXX variable is overwritten with an empy value when
host-autoconf-archive package has been installed before
host-ola.

config.log:
configure:19965:  -o conftest -O2 [snip -I and -L] conftest.cpp  -ldl

Fixes:
http://autobuild.buildroot.net/results/8fa/8fa0a64422a96023a926412a033d9a4f01af5a71/build-end.log



Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>