rpm-build: Support macros.d

[dmik]

I found that rpm-build.i386 version 4.8.1-22.oc00 doesn't seem to support automatically loading macros from the /@unixroot/usr/lib/rpm/marcos.d directory.

Some packages install various macros useful for maintaining .spec files. In particular, the texinfo package (info sub package) installs a file called macros.info that contains macros useful for handling documentation in Info format (%info_requires, %info_post, %info_preun) which save some monkey work and about to be used in nearly every .spec file (here is more info: ​http://trac.netlabs.org/rpm/wiki/RpmHowToPackagers#HandlingdocumentationinInfoformat).

We need to make this mechanism work.