Lower priority of the configure option --with-libraries causes wrong lib resolution

Hello,
I am having issues building Postgres due to the `./configure` script. I
found a workaround but I would appreciate a more reliable fix.

I use Arch Linux 64bit and want to build Postgres with the latest libicu
version which is version 77.1. Unfortunately the Arch package is still
at version 76 currently [0]https://archlinux.org/packages/core/x86_64/icu/. To solve this, I built libicu 77.1 locally.
I also built the latest libxml2 with the latest libicu to ensure compat.
This means that I have a local directory with shared objects and include
files for libicu 77. Let's say that these dependencies are in
`/postgres-deps/lib` and `/postgres-deps/include`. I also still have the
system-level ICU version 76 present in `/usr/lib`.

My goal is to build Postgres with the libicu version from `libicuuc.so`
(77) instead of the one from `/usr/lib` (76). Both directories contain a
file named `libicuuc.so`.

To achieve it, I called the `configure` script with the arguments
`--with-libraries` and `--with-includes`, as such: `./configure
--with-libraries=/postgres-deps/lib
--with-includes=/postgres-deps/include ...` (+ some extra `--with` flags
to enable features, irrelevant to this issue I think).

I then initiate the build with `LD_LIBRARY_PATH="/postgres-deps/lib "
make all`. The C compilation into object code succeeds using the libicu
77 includes, but then it fails to link with many errors such as:

```
/usr/bin/ld: commands/collationcmds.o: in function `get_icu_locale_comment':
collationcmds.c:(.text+0x1a20): undefined reference to
`uloc_getDisplayName_77'
```

The failing command is fairly long so I shortened it to focus on the
main part:

```

gcc [...COMPILER_FLAGS] [...OBJECT_FILES] -L../../src/port
-L../../src/common  -L/usr/lib -L/postgres-deps/lib -Wl,--as-needed 
-Wl,--export-dynamic -lzstd -llz4 -lxslt -lxml2 -lpam -lssl -lcrypto
-lgssapi_krb5 -lz -lm -lldap -licui18n -licuuc -lsystemd -o postgres
```

In particular, notice that the lib locations give priority to the system
directory instead of the `--with-libraries` directory that I passed to
`./configure`: `-L/usr/lib  -L/postgres-deps/lib`. Since the command
requests `-licuuc`, the file `/usr/lib/libicuuc.so` (version 76) is
matched first and the file `/usr/lib/libicuuc.so` (77) is ignored.

Swapping the two flags so the order is `-L/postgres-deps/lib -L/usr/lib`
fixes the build. I consider it very surprising that that libraries
passed with `--with-libraries` have lower priority, I would expect
explicitly requested libraries to have the highest priority.

I've searched a bit to find where both locations are inserted.

First, `/usr/lib` is added in the clang configuration [1]https://github.com/postgres/postgres/blob/6d6480066c1a96c7130b97b1139fdada9d484f80/configure#L5197. It is
retrieved from `/usr/bin/llvm-config --ldflags`.

Second, the `--with-libraries` directories are collected into
`$LIBDIRS`, with a check to verify that the dirs exists, this adds
`/postgres-deps/lib` [2]https://github.com/postgres/postgres/blob/6d6480066c1a96c7130b97b1139fdada9d484f80/configure#L8104.

Finally, $LIBDIRS is _appended at the end of LDFLAGS_.using
`LDFLAGS="$LDFLAGS $LIBDIRS"` [3]https://github.com/postgres/postgres/blob/6d6480066c1a96c7130b97b1139fdada9d484f80/configure#L9824.

My workaround is to update the configure script to instead prepend
`LIBDIRS` at the start of `LDFLAGS` using `LDFLAGS="$LIBDIRS $LDFLAGS"`.
With this change, the lib dir order is `-L/postgres-deps/lib -L/usr/lib`
and the version 77 of libicuuc.so is picked by the linker. The build
completes and I get a fully functional Postgres (at least it passes the
test suite of my application depending on it).

My expectation that moving the lib directories passed using
`--with-libraries` before any other linker flags makes sense, however
I'm not an expert in this area and I guess that there may be situations
where having the CLI libs first may cause issues. I can send a patch
swapping the application order or `LIBDIRS` as described in my
workaround if it makes sense. If there is a better solution to give
higher priority to the libs passed through the configure CLI, I would
gladly use it.

[0]: https://archlinux.org/packages/core/x86_64/icu/

[1]: https://github.com/postgres/postgres/blob/6d6480066c1a96c7130b97b1139fdada9d484f80/configure#L5197
https://github.com/postgres/postgres/blob/6d6480066c1a96c7130b97b1139fdada9d484f80/configure#L5197

[2]: https://github.com/postgres/postgres/blob/6d6480066c1a96c7130b97b1139fdada9d484f80/configure#L8104
https://github.com/postgres/postgres/blob/6d6480066c1a96c7130b97b1139fdada9d484f80/configure#L8104

[3]: https://github.com/postgres/postgres/blob/6d6480066c1a96c7130b97b1139fdada9d484f80/configure#L9824
https://github.com/postgres/postgres/blob/6d6480066c1a96c7130b97b1139fdada9d484f80/configure#L9824

Thank you in advance,
Charles "demurgos" Samborski