Derive the includes from config.h instead of define alloca
Derive the includes from config.h instead of define alloca

[r2109]

jonashaag/libfirm,davidgiven/libfirm,jonashaag/libfirm,MatzeB/libfirm,killbug2004/libfirm,MatzeB/libfirm,8l/libfirm,davidgiven/libfirm,killbug2004/libfirm,davidgiven/libfirm,MatzeB/libfirm,jonashaag/libfirm,davidgiven/libfirm,8l/libfirm,MatzeB/libfirm,libfirm/libfirm,jonashaag/libfirm,libfirm/libfirm,MatzeB/libfirm,killbug2004/libfirm,killbug2004/libfirm,8l/libfirm,8l/libfirm,davidgiven/libfirm,jonashaag/libfirm,killbug2004/libfirm,MatzeB/libfirm,8l/libfirm,davidgiven/libfirm,jonashaag/libfirm,killbug2004/libfirm,davidgiven/libfirm,libfirm/libfirm,libfirm/libfirm,killbug2004/libfirm,libfirm/libfirm,MatzeB/libfirm,jonashaag/libfirm,8l/libfirm,8l/libfirm