pinentry: fix build issue with gcc 5.x

pinentry: fix build issue with gcc 5.x

pinentry currently fails to build with gcc 5.x, because it uses some
functionality from std::string that requires C++11 without passing the
appropriate -std= value. We fix this by passing a proper -std= option
when gcc >= 5.x. Note that this can be removed once pinentry is bumped
to the newest version (currently 0.9.7) because they no longer use
std::string in the Qt part.

Fixes:

  http://autobuild.buildroot.net/results/6ea64f692acaebc4d58f3371c632b5121500f17e/
  (and many, many similar build failures)

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
One-beer-granted-to: Yann E. MORIN <yann.morin.1998@free.fr>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>