Enforce GNU 11 as default C dialect
Enforce GNU 11 as default C dialect

We do not set a C version level, leaving it to the compiler to enforce
whatever it saw fit.

We require GCC 4.8 or above, which supports C11, and GCC 5.2 and clang
3.7 default to this version of the standard, so set it as the default.

Signed-off-by: Joel Stanley <4816dabf8db1bc6cac35b3a24cab2ff844b5b0c7@jms.id.au>
[ec31ab75ddf977353c8f660f92ea8b23f64aef25@linux.vnet.ibm.com: rework to use new try-cflag magic]
Signed-off-by: Stewart Smith <ec31ab75ddf977353c8f660f92ea8b23f64aef25@linux.vnet.ibm.com>

mikey/skiboot,shenki/skiboot,apopple/skiboot,open-power/skiboot,qemu/skiboot,open-power/skiboot,qemu/skiboot,apopple/skiboot,apopple/skiboot,csmart/skiboot,shenki/skiboot,legoater/skiboot,legoater/skiboot,mikey/skiboot,shenki/skiboot,open-power/skiboot,legoater/skiboot,mikey/skiboot,legoater/skiboot,qemu/skiboot,open-power/skiboot,shenki/skiboot,qemu/skiboot,stewart-ibm/skiboot,csmart/skiboot,open-power/skiboot,shenki/skiboot,legoater/skiboot,stewart-ibm/skiboot,csmart/skiboot,qemu/skiboot,stewart-ibm/skiboot