Test if errno.h is available for some platforms

[Vincent Richomme]

I have also tried to cross-compile cegcc and I get a configure error in 
libphobos :

checking for snprintf... no
configure: error: No variant of snprintf.
make[1]: *** [configure-target-libphobos] Erreur 1


I don't understand how libphobos check if there is snprintf because 
other gcc lib (binutils, intl, libiberty, ...) can find it.