That should be only on the CFlags. When using the compiler
as the linker driver, one may want to also call pkg-config to get the
cflags, but when invoking the linker, there should be no compiler flags.
Currently, downstream libraries such as RProtoBuf fail because
`-std=c++11` is required via `configure.ac`, but RProtobuf has no way
of knowing this by consulting `pkg-config --cflags protobuf`.
