build: Check if we need to link against libargp

The build of `cockpit-tls` otherwise fails with a linker error on systems using
the musl C library:
```
[...]

  CCLD     cockpit-tls
ld.lld: error: undefined symbol: argp_parse
>>> referenced by main.c:103 (/home/marv/devel/cockpit/src/tls/main.c:103)
>>>               src/tls/main.o:(main)

ld.lld: error: undefined symbol: argp_error
>>> referenced by main.c:51 (/home/marv/devel/cockpit/src/tls/main.c:51)
>>>               src/tls/main.o:(arg_parse_int)
clang-15: error: linker command failed with exit code 1 (use -v to see invocation)
```

configure.ac

@@ -82,6 +82,13 @@ AC_ARG_ENABLE(old_bridge, AS_HELP_STRING([--enable-old-bridge], [Install old C c
 AM_CONDITIONAL(WITH_OLD_BRIDGE, test "$enable_old_bridge" = "yes")
 AC_MSG_RESULT(${enable_old_bridge:=no})
 
+AC_SEARCH_LIBS([argp_parse], [argp])
+case "$ac_cv_search_argp_parse" in
+    no) AC_MSG_FAILURE([failed to find argp_parse]) ;;
+    -l*) argp_LIBS="$ac_cv_search_argp_parse" ;;
+    *) argp_LIBS= ;;
+esac
+AC_SUBST([argp_LIBS])
 
 AC_SEARCH_LIBS([fts_close], [fts])
 case "$ac_cv_search_fts_close" in

src/tls/Makefile-tls.am

@@ -31,7 +31,7 @@ libcockpit_tls_a_SOURCES = \
 # cockpit-tls
 
 libexec_PROGRAMS += cockpit-tls
-cockpit_tls_LDADD = $(libcockpit_tls_a_LIBS)
+cockpit_tls_LDADD = $(libcockpit_tls_a_LIBS) $(argp_LIBS)
 cockpit_tls_SOURCES = src/tls/main.c
 
 # -----------------------------------------------------------------------------