configure: for C++11 support, at least GCC 4.6 is required

Change-Id: I0b61d738a85fc0d874fa9e18b4af4ebde5a40385
configure: another GCC version check that is always true

Change-Id: Iafbc0dadc1aba4267414e2c6653bf306e4d84f4c
gbuild: -Wnon-virtual-dtor and strict aliasing workaround for GCC < 4.6

... is obsolete, remove that.

Change-Id: I541de32ac4389d8049c25c98b63a69d084d1594d
gbuild: enable -Wnon-virtual-dtor for all GCC platforms

It was disabled globally at a time when GCC did not yet support #pragma
GCC diagnostic.

Change-Id: I062f6b437e6119203f2b0824a25238b45b4f4e79