(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.