configure script doesn't work on dash-only systems

[bedroge]

Describe the bug
While bumping the Lmod version in Gentoo (gentoo/gentoo#39798), it was reported that the configure script fails for a few people (also see https://bugs.gentoo.org/890939) who use dash for /bin/sh. It seems there are some dash-incompatible checks (e.g. https://github.com/TACC/Lmod/blob/main/configure#L4438 and https://github.com/TACC/Lmod/blob/main/configure#L4462, where the substring expansion is not supported on dash, but there are more).

I tried to reproduce this, and it seems like it only happens on systems with /bin/sh -> dash and which don't have bash at all (otherwise configure is picking up bash anyway?). That made me wonder if dash-only is supposed to be supported by Lmod? If not, maybe it would be nice if it could detect this and fail with a clear error message? If it is supported, some checks need to be changed to something that is supported by dash.

To Reproduce
Steps to reproduce the behavior:

Run ./configure on a system without bash installed, and with /bin/sh -> dash. This is a small reproducer that shows why it breaks in this case:

$ cat substring.sh
#!/bin/sh
PATH_TO_LUA=/some/path
#echo ${PATH_TO_LUA:0:1}
$ ./substring.sh
./substring.sh: 3: Bad substitution

Expected behavior
The ./configure script completes without errors or gives a clear error about dash-only not being supported.

Desktop (please complete the following information):

• OS: Linux
• Linux distribution: Gentoo/Ubuntu
• Lmod Version: 8.7.55

[bedroge]

I tried to change the lines that do not work for dash, and then the configure script does complete. The make install still failed because the proj_mgmt/convert_mode.sh and proj_mgmt/DATE_cmd.sh had a /bin/bash shebang, but changing that to /bin/sh solved it. The installation completed without issues and seems to work fine.

[bedroge]

This is a full diff of my configure (where I chose a printf solution to get the first character, https://stackoverflow.com/questions/27791251/get-first-character-of-a-string-shell has quite a few other ones) and the original one:

# diff configure.orig configure.new
4438c4438
<     if test ${PATH_TO_LUA:0:1} != '/' ; then
---
>     if test $(printf %.1s "$PATH_TO_LUA") != '/' ; then
4462c4462
<     if test ${PATH_TO_LUAC:0:1} != '/' ; then
---
>     if test $(printf %.1s "$PATH_TO_LUAC") != '/' ; then
4738c4738
< if test $MODE == "user_default" ; then
---
> if test $MODE = "user_default" ; then
5405,5406c5405
< 
< if ! command -v $PATH_TO_LUA &>/dev/null ; then
---
> if ! (command -v $PATH_TO_LUA >/dev/null 2>&1) ; then
5413c5412
<    if test "${PATH_TO_LUA:0:1}" = '/' -o "${PATH_TO_LUAC:0:1}" = '/'; then
---
>    if test $(printf %.1s "$PATH_TO_LUA") = '/' -o $(printf %.1s "$PATH_TO_LUA") = '/'; then
5423c5422
<    if test "${PATH_TO_LUA:0:1}" = '/' -a "${PATH_TO_LUAC:0:1}" != '/'; then
---
>    if test $(printf %.1s "$PATH_TO_LUA") = '/' -a $(printf %.1s "$PATH_TO_LUA") != '/'; then
5426c5425
<    if test "${PATH_TO_LUA:0:1}" != '/' -a "${PATH_TO_LUAC:0:1}" = '/'; then
---
>    if test $(printf %.1s "$PATH_TO_LUA") != '/' -a $(printf %.1s "$PATH_TO_LUA") = '/'; then
5436c5435
< if ! command -v $PATH_TO_LUA &>/dev/null ; then
---
> if ! (command -v $PATH_TO_LUA >/dev/null 2>&1) ; then
5442c5441
< if test ${PATH_TO_LUA:0:1} != '/' ; then
---
> if test $(printf %.1s "$PATH_TO_LUA") != '/' ; then
5491c5490
< if ! command -v $PATH_TO_LUAC &>/dev/null ; then
---
> if ! (command -v $PATH_TO_LUAC >/dev/null 2>&1) ; then
5497c5496
< if test "${PATH_TO_LUAC:0:1}" != '/' ; then
---
> if test $(printf %.1s "$PATH_TO_LUA") != '/' ; then
5592c5591
<   if ! $PATH_TO_LUA $ac_confdir/proj_mgmt/luaModuleAvailable $i 2> /dev/null ; then
---
>   if ! ($PATH_TO_LUA $ac_confdir/proj_mgmt/luaModuleAvailable $i >/dev/null) ; then