https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78110
--- Comment #1 from Bernd Edlinger <bernd.edlinger at hotmail dot de> --- oh, I am using eCos and that is based on newlib but on a very very old one. I don't know what happens when I dont use --with-newlib, but I can try... configure.ac has this: # First, test for "known" system libraries. We may be using newlib even # on a hosted environment. if test "x${with_newlib}" = "xyes"; then os_include_dir="os/newlib" AC_DEFINE(HAVE_HYPOT) ... AC_DEFINE(HAVE_ICONV) AC_DEFINE(HAVE_MEMALIGN) why are these not compile-time checks, to see if the headers are available?