investigate dtype test failures on non-Linux64 platforms

[TallJimbo]

The dtypes unit test seems quite fragile, and often fails when the library is otherwise working correctly. Problems have been confirmed on Mac OS and Windows, but have not been seen on Linux64.

The problem is probably related to the long-standing issue with multiple C/C++ types being mapped to the same NumPy type (or different NumPy types with the same name, so only one is accessible from Python), but the test itself may be at fault.

[rezahousseini]

I get errors on Arch Linux:

Running tests...
Test project /home/hous/code/Boost.NumPy/build
Start 1: dtype_mod
1/6 Test #1: dtype_mod ........................***Failed Error regular expression found in output. Regex=[ERROR:] 0.17 sec
Start 2: indexing_mod
2/6 Test #2: indexing_mod ..................... Passed 0.14 sec
Start 3: ndarray_mod
3/6 Test #3: ndarray_mod ...................... Passed 0.14 sec
Start 4: shapes_mod
4/6 Test #4: shapes_mod ....................... Passed 0.13 sec
Start 5: templates_mod
5/6 Test #5: templates_mod .................... Passed 0.14 sec
Start 6: ufunc_mod
6/6 Test #6: ufunc_mod ........................ Passed 0.14 sec

83% tests passed, 1 tests failed out of 6

Total Test time (real) = 0.86 sec

The following tests FAILED:
1 - dtype_mod (Failed)
Makefile:116: recipe for target 'test' failed

I don't see the problem. Incorporating dtype::get_builtin in my code causes a segfault.