fix gemspec files filter

fix build with XCode 4.3

- avoid use of FlatCarbon headers

- remove sysroot/syslibroot

- fix gemspec files attribute

skip rake cleans unless ruby/Makefile exists

SIGAR 1.6.4 release tag

sigar 1.6.3 release tag

sigar-1.6 tag

sigar 1.6.1 release tag

sigar 1.6.0 release tag