package/libabseil-cpp: enforce C++11

Abseil needs at least c++11 but removed the fallback when not set in [0].
So we need to do this.

[0] https://github.com/abseil/abseil-cpp/commit/c6954897f7ece5011f0126db9117361dc1a6ff36

Fixes:
  http://autobuild.buildroot.net/results/5780d5e21e5e9c0ba86cc422ef995f45dc83cc4c
  http://autobuild.buildroot.net/results/8138c63fc13940e1b97f07f24752b199f88ccfa8
  http://autobuild.buildroot.net/results/f8eda966c6400c242680db378c8944444d89d5c5
  http://autobuild.buildroot.net/results/38e9427d995e16bc4876f9fdc890c42e4fd04c06
  http://autobuild.buildroot.net/results/4e99415728a6f3c6b396a9872924262652f77996
  http://autobuild.buildroot.net/results/1830577843772bdfd6e310b4d361aa1edf5969d6



Signed-off-by: Michael Nosthoff <buildroot@heine.tech>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>