(maint) Specifically link against rt library

Address failure:
`../libfacter.so.0.2.0: undefined reference to `clock_gettime'
collect2: error: ld returned 1 exit status`

It looks like we are incorrectly linking against the rt library. In
newer el versions, it is likely that this gets pulled correctly, but for
older versions of el, this doesn't seem to be the case.
(CFACT-156) Build CFacter for el5

Unfortunately, el5 doesn't have any of the "optional" buildtime
requirements. It does have curl and openssl development packages
available, however, these are both of insufficient versions (curl 7.15.5
and openssl 0.9.8e as opposed to curl >= 7.18.0 and openssl >= 1.0.1g).
In light of this, we will not build with these dependencies for el5.
(maint) Allow for easier building on AIX

This is something to help us in the future. Eventually, we will use this
spec file to build packages for AIX. Since AIX is very particular about
the syntax is supports, these are some very small changes to comply with
that. Nested conditionals are a no no. Obviously, this is only the tip
of the iceberg when it comes to changes that will be needed for AIX, but
it's nice to have one less thing to think about when the time comes
Merge pull request #212 from melissaanne/master

Update for building cfacter for el5