configure.ac: fix build without dlfcn.h

configure.ac: fix build without dlfcn.h

Fix the following static build failure (i.e. without dlfcn.h) raised
since version 1.2.7 and
https://github.com/alsa-project/alsa-utils/commit/44d3e8aa44fc12f95bfeef0e3a1051e8f77047b5:

pre-processor.c:28:10: fatal error: dlfcn.h: No such file or directory
   28 | #include <dlfcn.h>
      |          ^~~~~~~~~

Fixes:
 - http://autobuild.buildroot.org/results/1173a1ee28a58ce565a3274f35e868c35bb2e04e

Fixes: https://github.com/alsa-project/alsa-utils/pull/165
Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>