compile: Pass --rcfile /etc/rpmrc to rpmspec

By default, the vendored rpm-4.11.1 uses its own macros rather than
the system's macros from /etc/rpmrc. This causes problems because the
gluster specfile assumes %filter_provides_in is defined on el6, even
though it was superceeded in rpm-4.11.1, resulting in the error:

    Unknown tag:     %filter_provides_in