IRIX/MipsPro patches from Erik Hofman:

IRIX/MipsPro patches from Erik Hofman:

This patch adds some missing default ISO C++ headers to SimGear,
especially usefull for Irix/MipsPro. To the best of my knowledge this
only affects the Irix/MipsPro compiler combination although it might be
usefull for others as well.

This patch does not yet remove any compiler specific code, but that
would be my next task. The powerfull macros as defined in the compiler.h
file allowed me to split it up into multiple patches.