Changes to get SimGear to configure and compile out-of-the-box on

Changes to get SimGear to configure and compile out-of-the-box on
a MinGW target:

Link against alut.dll in addition to openal32.dll.

Remove some preprocessor defines from compiler.h that were
confusing the mingw and/or libstdc++ headers (I put the _isnan
one back in the only file it was used).

Hack a broken sleep() call into the OpenAL sample programs so
that they will compile (but not work) in a non-POSIX environment.

Change the header file ordering in sample_openal.hxx to get
around some really weird interactions between MinGW's windows.h
and the gcc iostream header.