[C++] Do not build HDFS IO interface optionally

[asfimport]

Currently if you have not built arrow::io with -DARROW_HDFS=on then importing pyarrow.parquet fails with a not very graceful error message from failure to find the right symbols in libarrow_io.so. Since we vendor hdfs.h and the libraries are loaded at runtime, having this extra option doesn't add much value.

Reporter: Wes McKinney / @wesm
Assignee: Wes McKinney / @wesm

_{Note: This issue was originally created as ARROW-322. Please see the migration documentation for further details.}

[asfimport]

Uwe Korn / @xhochy:
That also implies that we should have those module as separate shared libraries in arrow-cpp?

[asfimport]

Wes McKinney / @wesm:
@xhochy since there's no extra thirdparty dependency and these libraries are loaded only at runtime using dlopen, there's little harm in always building the module. If it becomes an issue for some reason in the future, we can split it out into a leaf shared library

[asfimport]

Wes McKinney / @wesm:
PR: #253

[asfimport]

Uwe Korn / @xhochy:
Issue resolved by pull request 253
#253