When linking with C++ files the linker also needs to link against the C++
libraries. This is done automatically when invoking the compiler upon linking.
Since we don't want C++ dependencies on C-only projects we check if we actually
have C++ files and use either the C or C++ compiler.
Rename CFLAGS since it's now used for both C and C++ compiler and add dedicated
CFLAGS, CXXFLAGS and LDFLAGS variables.
Change-Id: I9cc068a8038f21e8fd96b20173a8f790e6ab4b6e
3ee79724f6