I don't personally use Ubuntu (rather than UNIX/*BSD/etc., Slackware GNU/Linux) but you mentioned it, so I'm trying to build on family PC's Kubuntu-based GNU/Linux (KDE Neon, which is like Kubuntu 20.04 with newer KDE, no other changes. Of course Kubuntu itself is on to 22.04) It worked for a version in 2021 but now says the below.
root@Hilltop-Heaven:~/math/src/ncsa-mosaic# apt-get install build-essential libmotif-dev libjpeg62-dev libpng12-dev x11proto-print-dev libxmu-headers libxpm-dev libxmu-dev
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package libpng12-dev is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
E: Package 'libpng12-dev' has no installation candidate
E: Unable to locate package x11proto-print-dev
root@Hilltop-Heaven:~/math/src/ncsa-mosaic# make linux
...
gcc -g -DMOTIF1_2 -DLINUX -DPRERELEASE -Wall -Wno-parentheses -Wno-switch -Wno-switch-enum -Wno-char-subscripts -Wno-unused-result -I/usr/X11R6/include -DHAVE_JPEG -DHAVE_PNG -I.. -I../libXmx -I../libwww2 -c -o main.o main.c
Xmx.c:59:10: fatal error: Xm/RepType.h: No such file or directory
59 | #include <Xm/RepType.h>
| ^~~~~~~~~~~~~~
compilation terminated.
make[2]: *** [<builtin>: Xmx.o] Error 1
make[2]: *** Waiting for unfinished jobs....
gcc -g -DMOTIF1_2 -DLINUX -DPRERELEASE -Wall -Wno-parentheses -Wno-switch -Wno-switch-enum -Wno-char-subscripts -Wno-unused-result -I/usr/X11R6/include -c -o HTFTP.o HTFTP.c
In file included from HTMLlists.c:56:
HTML.h:59:10: fatal error: Xm/Xm.h: No such file or directory
59 | #include <Xm/Xm.h>
| ^~~~~~~~~
compilation terminated.
In file included from HTMLP.h:58,
from HTML.c:56:
HTML.h:59:10: fatal error: Xm/Xm.h: No such file or directory
59 | #include <Xm/Xm.h>
| ^~~~~~~~~
compilation terminated.
make[2]: *** [<builtin>: HTMLlists.o] Error 1
make[2]: *** Waiting for unfinished jobs....
gcc -g -DMOTIF1_2 -DLINUX -DPRERELEASE -Wall -Wno-parentheses -Wno-switch -Wno-switch-enum -Wno-char-subscripts -Wno-unused-result -I/usr/X11R6/include -c -o HTFWriter.o HTFWriter.c
make[2]: *** [<builtin>: HTML.o] Error 1
make[2]: Leaving directory '/root/math/src/ncsa-mosaic/libhtmlw'
make[1]: *** [makefiles/Makefile.linux:266: libhtmlw] Error 2
make[1]: *** Waiting for unfinished jobs....
gcc -g -DMOTIF1_2 -DLINUX -DPRERELEASE -Wall -Wno-parentheses -Wno-switch -Wno-switch-enum -Wno-char-subscripts -Wno-unused-result -I/usr/X11R6/include -c -o HTFile.o HTFile.c
HTFTP.c:42:10: fatal error: X11/Intrinsic.h: No such file or directory
42 | #include <X11/Intrinsic.h>
| ^~~~~~~~~~~~~~~~~
compilation terminated.
make[2]: *** [<builtin>: HTFTP.o] Error 1
make[2]: *** Waiting for unfinished jobs....
In file included from XmxP.h:98,
from Xmx2.c:57:
Xmx.h:61:10: fatal error: X11/Intrinsic.h: No such file or directory
61 | #include <X11/Intrinsic.h>
| ^~~~~~~~~~~~~~~~~
compilation terminated.
make[2]: *** [<builtin>: Xmx2.o] Error 1
make[2]: Leaving directory '/root/math/src/ncsa-mosaic/libXmx'
make[1]: *** [makefiles/Makefile.linux:262: libXmx] Error 2
In file included from ../src/mosaic.h:102,
from HTAlert.c:18:
../src/../libXmx/Xmx.h:61:10: fatal error: X11/Intrinsic.h: No such file or directory
61 | #include <X11/Intrinsic.h>
| ^~~~~~~~~~~~~~~~~
compilation terminated.
In file included from ../src/mosaic.h:102,
from HTAccess.c:44:
../src/../libXmx/Xmx.h:61:10: fatal error: X11/Intrinsic.h: No such file or directory
61 | #include <X11/Intrinsic.h>
| ^~~~~~~~~~~~~~~~~
compilation terminated.
make[2]: *** [<builtin>: HTAlert.o] Error 1
make[2]: *** [<builtin>: HTAccess.o] Error 1
In file included from mosaic.h:102,
from main.c:61:
../libXmx/Xmx.h:61:10: fatal error: X11/Intrinsic.h: No such file or directory
61 | #include <X11/Intrinsic.h>
| ^~~~~~~~~~~~~~~~~
In file included from HTFWriter.c:23:
../libhtmlw/HTML.h:65:10: fatal error: X11/Intrinsic.h: No such file or directory
65 | #include <X11/Intrinsic.h>
| ^~~~~~~~~~~~~~~~~
compilation terminated.
compilation terminated.
make[2]: *** [<builtin>: main.o] Error 1
make[2]: Leaving directory '/root/math/src/ncsa-mosaic/src'
make[2]: *** [<builtin>: HTFWriter.o] Error 1
make[1]: *** [makefiles/Makefile.linux:278: src] Error 2
make[2]: Leaving directory '/root/math/src/ncsa-mosaic/libwww2'
make[1]: *** [makefiles/Makefile.linux:270: libwww2] Error 2
make[1]: Leaving directory '/root/math/src/ncsa-mosaic'
make: *** [Makefile:67: linux] Error 2