Changes to get FlightGear (well, the src directory at least) to

Changes to get FlightGear (well, the src directory at least) to
configure and compile out-of-the-box on a MinGW target:

Use -lSDL instead of -lglut32 on windows builds when --enable-sdl
is set.

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

Replace BSD bcopy() with ANSI C memmove() in a few places.  This is
simpler than trying to abstract it out as a platform dependency in a
header file; bcopy() has never been standard.

The ENABLE_THREADS handling has changed to be set to 0 when threads
are not in use.  This breaks expressions like #ifdef ENABLE_THREADS.
Replace with a slightly more complicated expression.  It might have
been better to fix the configure.ac script, but I didn't know how and
this whole setting is likely to go away soon anyway.

The MinGW C runtime actually does include snprintf, so only MSVC
builds (and not all WIN32 ones) need _snprintf in JSBSim/FGState.cpp

Building on a platform with no glut at all exposed some spots where
plib/pu.h was being included without a toolkit setting (it defaults to
glut).  Include fg_os.hxx first.

And when still using glut, glut.h has a bizarre dependency on a
_WCHAR_T_DEFINED symbol.  It it's not defined, it tries to redefine
(!!) wchar_t to disasterous effect.