link to correct msvcr using compiler specs

msvcrt.dll is used as default runtime library by mingw32, currently numpy.distutils tries to replace it by setting -D__MSVCRT_VERSION__=0x0900 and -lmsvcr90. In my case this creates additional (correct) dependency on msvcr90.dll but does not remove msvcrt.dll.

To achieve correct linking (only) to msvcr90.dll, compiler specs need to be modified, since linking to msvcrt.dll happens implicitly in the internals. (look at the first comment here: http://www.mingw.org/wiki/HOWTO_Use_the_GCC_specs_file#comment-106)

This was also reported as issue #4368