glibc/sysdeps/unix/sysv/linux/configure.in

sinclude(./aclocal.m4)dnl Autoconf lossage
GLIBC_PROVIDES dnl See aclocal.m4 in the top level source directory.
# Local configure fragment for sysdeps/unix/sysv/linux.

# On Linux, the default is to use libio instead of stdio.
test $stdio = default && stdio=libio

# Don't bother trying to generate any glue code to be compatible with the
# existing system library, because we are the only system library.
inhibit_glue=yes

define([LIBC_LINUX_VERSION],[2.0.10])dnl
define([libc_cv_linuxVER], [libc_cv_linux]patsubst(LIBC_LINUX_VERSION,[\.]))dnl
AC_CACHE_CHECK(installed Linux kernel header files, libc_cv_linuxVER, [dnl
AC_TRY_COMPILE([#include <linux/version.h>],
[#if LINUX_VERSION_CODE < ]dnl
patsubst(LIBC_LINUX_VERSION,[^\([^.]*\)\.\([^.]*\)\.\([^.]*\)$],dnl
[ (\1 *65536+ \2 *256+ \3) /* \1.\2.\3 */])[
eat flaming death
#endif],
	       libc_cv_linuxVER='LIBC_LINUX_VERSION or later',
	       libc_cv_linuxVER='TOO OLD!')])
if test "$libc_cv_linuxVER" != 'LIBC_LINUX_VERSION or later'; then
  AC_MSG_ERROR([GNU libc requires kernel header files from
Linux LIBC_LINUX_VERSION or later to be installed before configuring.
The kernel header files are found usually in /usr/include/asm and
/usr/include/linux; make sure these directories use files from
Linux LIBC_LINUX_VERSION or later.  This check uses <linux/version.h>, so
make sure that file was built correctly when installing the kernel header
files.])
fi

# The Linux filesystem standard prescribes where to place "essential"
# files.  I.e., when the installation prefix is "/usr" we have to place
# shared library objects and the configuation files on the root partition
# in /lib and /etc.
if test "$prefix" = "/usr"; then
  libc_cv_slibdir="/lib"
  libc_cv_sysconfdir="/etc"
fi

# Under Linux the LinuxThreads and crypt add-on should be available.
case $add_ons in
  # Both are available.  Good.
  *linuxthreads*des-crypt* | *des-crypt*linuxthreads*)
    message=
    ;;
  *linuxthreads*)
    message="\
*** WARNING:
*** Are you sure you do not want to use the \`crypt' add-on?"
    ;;
  *des-crypt*)
    message="\
*** WARNING:
*** Are you sure you do not want to use the \`LinuxThread' add-on?"
    ;;
  *)
    message="\
*** WARNING: Are you sure you do not want to use the \`LinuxThreads'
*** and \`crypt' add-ons?"
    ;;
esac

if test "$message"; then
  if test $enable_sanity = yes; then
    echo "\
*** You should not compile the GNU libc without the \`LinuxThreads' and
*** \`crypt' add-on.  Not using them risks to be incompatible with the
*** libraries of other systems.  Consider getting the add-ons and restart
*** the configuration.
*** If you really mean to avoid those add-ons run configure again, now
*** using the extra parameter \`--disable-sanity-checks'."
    exit 1
  else
    echo "$message"
  fi
fi