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`.

The referenced bug was fixed in 2007 and has been released in pkg-config-0.23
(16.Jan.2008). The fixed version is widely available.