libebook-contacts: Always use the C linker

libebook-contacts: Always use the C linker

Ensure that libebook-contacts always gets linked using the C linker,
instead of the C++ automake would prefer, because of the optional
e-phone-number-private.cpp file. Without this change linking with
explicitly disabled phone number support (--without-phonenumber)
would fail because no C++ linker is know. Similarly the library
would depend on the C++ standard library if phone number was
disabled automatically, because libphonenumber was not found.