build: Bump C++ standard requirement to C++14 to fix FTBFS with LLVM 10

When building Mesa against a recent LLVM 10 with C++11, the build fails
if the AMD common code is built as well due to "std::index_sequence"
being undeclared.

LLVM requires a minimum of C++14.

Signed-off-by: Kai Wasserbäch <kai@dev.carbon-project.org>
Acked-by: Eric Engestrom <eric@engestrom.ch>
4 files changed