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