configure --with-glibc-be-dir override is silently ignored

Nobody has claimed this yet.

### Bug:
The --with-glibc-be-dir=DIR configure option is documented as the supported method for specifying a non-standard back-end glibc path on heterogeneous clusters.

The option is silently ignored at configure time due to an argument-processing ordering bug.

Detection block:
https://github.com/llnl/Spindle/blob/ba9bbcc84de7f66ed10dfa45e266c7d8aecd0c85/configure.common.ac#L374-L379

But argument parsing happens 40 lines later and GLIBC_BE_DIR is already set, so the assignment `GLIBC_BE_DIR=${withval}` has no effect
https://github.com/llnl/Spindle/blob/ba9bbcc84de7f66ed10dfa45e266c7d8aecd0c85/configure.common.ac#L416-L418

### Steps to recreate:

```
cd containers/spindle-serial-ubuntu
docker compose build
docker compose up -d
docker exec spindlenode grep 'GLIBC_BE_DIR' /home/spindleuser/Spindle-build/src/client/Makefile
```
```
GLIBC_BE_DIR = /lib/aarch64-linux-gnu
```

Reconfigure with glibc being set to a false path:
```
docker exec spindlenode bash -c '
cd ~/Spindle-build
../../Spindle/configure \
--prefix=~/Spindle-inst \
--enable-sec-munge \
--with-rm=serial \
--with-localstorage=/tmp \
--with-glibc-be-dir=/definitely/not/real \
2>&1 | grep -i glibc
'
docker exec spindlenode grep 'GLIBC_BE_DIR' /home/spindleuser/Spindle-build/src/client/Makefile
```
```
GLIBC_BE_DIR = /lib/aarch64-linux-gnu
```

### Fix:

Move `AC_ARG_WITH(glibc-be-dir, ...)` before the detection block so the `if test "x$GLIBC_BE_DIR" = "x"` guard correctly short-circuits when a user provides an explicit path.

Start in configure.common.ac at the glibc back-end detection block around lines 374-379 and the AC_ARG_WITH(glibc-be-dir) handling around lines 416-418. Rebuild or reconfigure using the commands in the issue, then inspect src/client/Makefile; done means an explicitly supplied path such as /definitely/not/real is preserved in GLIBC_BE_DIR instead of being replaced by detected glibc.

Written by the indexing model from the issue text.