Predef always fails on Windows with build-dir directive

Hi Rene,

I'm trying to use predef in my Boost.Build project but have run into a strange issue. The check programs like "predef_check_as_cpp.exe" are supposed to return 1 when the check is false, right? I created a test case showing some strange issues on Windows:

Jamroot.jam:

```
import path ;

path-constant BUILD_PATH : build ;

import predef/check/predef : require check : predef-require predef-check ;

project pwiz
: requirements
[ predef-check "BOOST_COMP_GNUC >= 4.8" : : -Wno-unused-local-typedefs ]
[ predef-check "BOOST_COMP_GNUC >= 4.9" : : -fdiagnostics-color=always ]

# comment out the next line and predef works on Windows; on Linux it works either way
: build-dir $(BUILD_PATH)
: default-build release
;

exe predef_test : predef_test.cpp ;
```

On Windows/MSVC-12, that's causing this:

```
predef.predef_check_action check\predef\bin\msvc-12.0\debug\threading-multi\3567d106897dec7fe92d032e763161c1.txt

check\predef\bin\msvc-12.0\debug\threading-multi\predef_check_as_cpp.exe check\predef\bin\msvc-12.0\debug\threading-multi\predef_check_as_cpp.pdb "BOOST_COMP_GNUC >= 4.8" > check\predef\bin\msvc-12.0\debug\threading-multi\3567d106897dec7fe92d032e763161c1.txt

...failed predef.predef_check_action check\predef\bin\msvc-12.0\debug\threading-multi\3567d106897dec7fe92d032e763161c1.txt...
...removing check\predef\bin\msvc-12.0\debug\threading-multi\3567d106897dec7fe92d032e763161c1.txt
...failed updating 1 target...
Performing configuration checks

- BOOST_COMP_GNUC >= 4.8 : no
...found 1 target...
...updating 1 target...
predef.predef_check_action check\predef\bin\msvc-12.0\debug\threading-multi\150e0cfdde3cea25efb86d81e179ac7a.txt

check\predef\bin\msvc-12.0\debug\threading-multi\predef_check_as_cpp.exe check\predef\bin\msvc-12.0\debug\threading-multi\predef_check_as_cpp.pdb "BOOST_COMP_GNUC >= 4.9" > check\predef\bin\msvc-12.0\debug\threading-multi\150e0cfdde3cea25efb86d81e179ac7a.txt

...failed predef.predef_check_action check\predef\bin\msvc-12.0\debug\threading-multi\150e0cfdde3cea25efb86d81e179ac7a.txt...
...removing check\predef\bin\msvc-12.0\debug\threading-multi\150e0cfdde3cea25efb86d81e179ac7a.txt
...failed updating 1 target...
- BOOST_COMP_GNUC >= 4.9 : no
...found 1 target...
...updating 1 target...
config-cache.write C:\pwiz-src\trunk\pwiz\predef-test\build-nt-x86_64\project-cache.jam
...updated 1 target...
...found 16 targets...
```

You can see it's building as debug even if I build the rest of the project is building as release, and it's also passing the pdb to the check command. If I edit predef.jam to build as release, then it still fails, but this time it fails because the return code is 1. I don't understand that. The predef.jam code is pretty cryptic relative to my Boost.Build skills, so I'm not sure what's going on. If I get rid of the build-dir directive, it builds fine on Windows (although it's still debug). It builds fine either way on Linux.