libsigrok: depends on gcc >= 4.7

Until recently, only the C++ bindings of libsigrok needed a recent
compiler because they are written in C++11. However, now, libsigrok
itself is written in C11, which is only available since gcc 4.7.

So, this commit replaces the CodeSourcery-specific exclusions by a
proper dependency on gcc >= 4.7.

The sigrok-cli and pulseview packages, which select libsigrok, are
also updated accordingly.

Fixes:
  http://autobuild.buildroot.org/results/1d7/1d75497009f1e3b06236b3409fd768dcf7956b87/
  http://autobuild.buildroot.org/results/563/563378e3f6320980153c8c972ceba5e913fe933f/



[Thomas: add autobuilder references, as suggested by Yann E. Morin.]

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