Use source built libpurple.la when building gir files

When running g-ir-scanner to generate Pidgin-3.0.gir, if the
installed libpurple.so didn't match the symbols in the built
version, it could fail. This is because g-ir-scanner was linking
to the installed version instead of the built one.

This patch makes it look at the pkg-config file to find the
right version, which in this case is pointed to by the uninstalled
pc file. I expected the --include-uninstalled flag to be what was
needed for this, but I couldn't get it to work.

Additionally, this patch sets the package name for the
Purple-3.0.gir for completeness.