Build failure with disabled SNMP support

[jtru]

Describe the bug
When trying to build ltfs as of today's current master on Debian 11/amd64 with --disable-snmp passed to configure, ld fails due to libdl not getting linked via AM_LDFLAGS.

To Reproduce
Steps to reproduce the behavior:

1. git clone 'https://github.com/LinearTapeFileSystem/ltfs.git' && cd ltfs
2. ./autogen.sh
3. ./configure --disable-snmp
4. make

Expected behavior
The build succeeds and yields binaries/artifacts without SNMP support.

Desktop (please complete the following information):

• OS: Debian 11 amd64

Additional context

In configure.ac, when building with SNMP support, AM_LDFLAG is ultimately expanded to include the output of the net-snmp-config --agent-libs command, which on my system happens to include the required -ldl argument for the linker to succeed.

I think passing that argument to all build steps where the linker targets objects that had their corresponding source file(s) (at least src/libltfs/ltfslogging.c and src/libltfs/plugin.c) use libdl features should fix the problem.

FWIW, as a workaround, I configured the source with LIBS=-ldl ./configure --disable-snmp, which succeeds.

[piste-jp]

In my RHEL7 env, -ldl is included into the configuration of ICU.

ICU_MODULE_CFLAGS=' -D_REENTRANT    -I/usr/include '
ICU_MODULE_LIBS='-Wl,-z,relro   -lpthread -ldl -lm   -L/usr/lib64 -licui18n -licuuc -licudata  -lpthread -ldl -lm   '

Did you use custom /usr/bin/icu-config described into #153? Or does Debian11 provide /usr/bin/icu-config into ICU packages?

[jtru]

Debian 11 does not provide icu-config:

$ dpkg -S icu-config
dpkg-query: no path found matching pattern *icu-config*

However, I did not prepare my build env (my desktop machine) in any way to build ltfs. Both configuration and build phase work for me without icu-config present, with the above-mentioned limitations when disabling SNMP support.

[piste-jp]

I realized package data for icu-uc is provided in Debian11. So configure script fetch the options from pkg-config.

I will fix the configure.ac to always include -ldl into AM_LDFLAGS. Because it is required in anytime (on Linux).

[jtru]

I can confirm current master builds successfully on Debian 11. Thanks very much for the fix!