/usr/bin/xmkmf is in xutils-dev 1:7.7+5ubuntu1.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 | #!/bin/sh
# make a Makefile from an Imakefile from inside or outside the sources
usage="usage:  $0 [-a] [top_of_sources_pathname [current_directory]]"
configdirspec="-I/usr/lib/X11/config"
topdir=
curdir=.
do_all=
imake_defines=
while [ $# -gt 0 ]
do
    case "$1" in
    -D*)
        imake_defines="$imake_defines $1"
 shift
 ;;
    -a)
 do_all="yes"
 shift
 ;;
    *)
     break
 ;;
    esac
done
case $# in
    0) ;;
    1) topdir=$1 ;;
    2) topdir=$1 curdir=$2 ;;
    *) echo "$usage" 1>&2; exit 1 ;;
esac
case "$topdir" in
    -*) echo "$usage" 1>&2; exit 1 ;;
esac
if [ -f Makefile ]; then
    echo mv -f Makefile Makefile.bak
    mv -f Makefile Makefile.bak
fi
if [ "$topdir" = "" ]; then
    args="-DUseInstalled "$configdirspec
else
    args="-I$topdir/config/cf -DTOPDIR=$topdir -DCURDIR=$curdir"
fi
echo imake $imake_defines $args
case "$do_all" in
yes)
    imake $imake_defines $args &&
    echo "make Makefiles" &&
    make Makefiles &&
    echo "make includes" &&
    make includes &&
    echo "make depend" &&
    make depend
    ;;
*)
    imake $imake_defines $args
    ;;
esac
 |