Use the visibility attributes only when building a shared library.

Use the visibility attributes only when building a shared library.

Omair Eshkenazi has reported that libpci fails to build with gcc 3.x.
We require gcc 4.x for building shared libraries, but it should not
be needed when SHARED=no.

This patch changes internal.h to define PCI_ABI as the visibility
attribute only if we build a shared library.

However, this means that we need to know the configuration settings
before we include pci.h, so I had to include config.h explicitly
from internal.h and create a new symbol PCI_CONFIG_H to check whether
config.h has been already included.