Optionally unavailable with uClibc-ng.

Switched to local tarballs. .xz is smaller.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

sigar is a System Information Gatherer And Reporter library for C++
Adding the package so other C++ packages that depends on this library
can build.
This creates a libsigar.so shared library.

Signed-off-by: Amol Bhave <ambhave@fb.com>