[C++] Also build static libraries for leaf libraries

[asfimport]

libarrow_io should also build a static library to give linkers the option to use either variant

Reporter: Wes McKinney / @wesm
Assignee: Uwe Korn / @xhochy

_{Note: This issue was originally created as ARROW-303. Please see the migration documentation for further details.}

[asfimport]

Uwe Korn / @xhochy:
PR: #255

[asfimport]

Wes McKinney / @wesm:
Resolved by 8aab00e