build: allow building cli without dynamic librarires support

Commit 3cb28534d34392ceec4adead0cfa97039796ccb7 enables building
of cli always as part of `make check`. As cli previously always
included <dlfcn.h>, this broke tests for building with toolchains
that don't support dynamic library loading.

Add a configure check and disable dlopen() based on whether
<dlfcn.h> is available.

Signed-off-by: Thomas Haller <thaller@redhat.com>

https://github.com/thom311/libnl/pull/141
diff --git a/configure.ac b/configure.ac
index e7402fd..387a53b 100644
--- a/configure.ac
+++ b/configure.ac
@@ -101,6 +101,8 @@
 AM_CONDITIONAL([ENABLE_CLI_INSTALL_BIN], [test "$enable_cli" = "bin"])
 AM_CONDITIONAL([ENABLE_CLI_INSTALL_SBIN], [test "$enable_cli" = "sbin"])
 
+AC_CHECK_HEADERS(dlfcn.h, [], [])
+
 AC_ARG_ENABLE([pthreads],
 	AS_HELP_STRING([--disable-pthreads], [Disable pthreads support]),
 	[enable_pthreads="$enableval"], [enable_pthreads="yes"])