Support pkg-config --static flag in capstone.pc

[michaeldleslie]

Adds support for pkg-config's --static flag so that downstream users don't need to manually specify the path to libcapstone.a when statically linking.

Before change:

$ pkg-config --libs capstone
-lcapstone
$ pkg-config --libs capstone --static
-lcapstone

After change:

$ pkg-config --libs capstone
-lcapstone
$ pkg-config --libs capstone --static
-lcapstone -l:libcapstone.a

[kabeor]

Cool, thanks! Merged.