BUILD: Fix Clang builds

Clang does not handle the -std=c++11 flag for C file - we had to
explicitly flag the compiler to build them as a C standard.

Signed-off-by: G4m4 <misept.dieseneuf@gmail.com>
BUILD: Add the "efficient C++ warnings"	flag

This checks for various common C++ issues.

Signed-off-by: G4m4 <misept.dieseneuf@gmail.com>