* Add -lkstat to the .pc for Solaris
This fixes linking for projects that rely on pkg-config to generate the
link line on Solaris.
Test plan: Built the project locally on Solaris and verified -kstat
appears in the .pc file
```
$ cat lib/pkgconfig/benchmark.pc | grep Libs.private
Libs.private: -lpthread -lkstat
```
* Use BENCHMARK_PRIVATE_LINK_LIBRARIES
Since pthread is required at least for GCC (according to the
documentation), this should be reflected by the pkg-config file.
The same is, for instance, also done by the gflags library:
1005485222/cmake/package.pc.in (L13)
