(maint) Skip libcurl on Solaris for now

For facter it's only used on linux (for query gce and ec2 metadata
endpoints), and we may be rolling our own libcurl as part of toolchain
work for Solaris, so we can sort this issue out later.
Merge pull request #199 from kylog/use-curl-only-on-linux-for-now

(maint) Skip libcurl on Solaris for now