cppcms: fix build with ICU

cppcms: fix build with ICU

Since the bump of ICU to 59.1, it needs C++11. cppcms has been updated
with the appropriate dependency on gcc >= 4.8, but it also needs
-std=c++11. Therefore, this patch complements the CXXFLAGS passed to
cppcms by adding what icu-config reports, in a manner similar to what
was done to fix the same issue in the PHP package in commit
214a76d045f906c1a8d687b7c7e7bc85c5b09906 ("package/php: fix icu
support").

Fixes:

  http://autobuild.buildroot.net/results/f05d9999f1f3248b6a50f14de476108028ed047e/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>