pdal_test fails to link on MSVC2012

A relatively minimal [1] build of PDAL master on MSVC2012 results in a couple of unresolved externals.

```
4>Link:
4>     Creating library C:/Users/chambersbj/TILT/pdal/build/msvc2012/lib/pdal_test.lib and object C:/Users/chambersbj/TILT/pdal/build/msvc2012/lib/pdal_test.exp
4>BPFTest.obj : error LNK2001: unresolved external symbol "protected: virtual class std::fpos<int> __cdecl pdal::Charbuf::seekpos(class std::fpos<int>,int)" (?seekpos@Charbuf@pdal@@MEAA?AV?$fpos@H@std@@V34@H@Z)
4>BPFTest.obj : error LNK2001: unresolved external symbol "protected: virtual class std::fpos<int> __cdecl pdal::Charbuf::seekoff(__int64,int,int)" (?seekoff@Charbuf@pdal@@MEAA?AV?$fpos@H@std@@_JHH@Z)
4>C:\Users\chambersbj\TILT\pdal\build\msvc2012\bin\pdal_test.exe : fatal error LNK1120: 2 unresolved externals
```

[1] WITH_APPS, WITH_GDAL, WITH_GEOTIFF, WITH_TESTS, WITH_ZLIB