configure: check if dlfcn.h is present for ladspa and frei0r, if not abort early

Fixes #4517.

configure

@@ -6065,11 +6065,11 @@ enabled chromaprint       && require chromaprint chromaprint.h chromaprint_get_v
 enabled decklink          && { require_headers DeckLinkAPI.h &&
                                { test_cpp_condition DeckLinkAPIVersion.h "BLACKMAGIC_DECKLINK_API_VERSION >= 0x0a090500" || die "ERROR: Decklink API version must be >= 10.9.5."; } }
 enabled libndi_newtek     && require_headers Processing.NDI.Lib.h
-enabled frei0r            && require_headers frei0r.h
+enabled frei0r            && require_headers "frei0r.h dlfcn.h"
 enabled gmp               && require gmp gmp.h mpz_export -lgmp
 enabled gnutls            && require_pkg_config gnutls gnutls gnutls/gnutls.h gnutls_global_init
 enabled jni               && { [ $target_os = "android" ] && check_headers jni.h && enabled pthreads || die "ERROR: jni not found"; }
-enabled ladspa            && require_headers ladspa.h
+enabled ladspa            && require_headers "ladspa.h dlfcn.h"
 enabled libaom            && require_pkg_config libaom "aom >= 1.0.0" aom/aom_codec.h aom_codec_version
 enabled lv2               && require_pkg_config lv2 lilv-0 "lilv/lilv.h" lilv_world_new
 enabled libiec61883       && require libiec61883 libiec61883/iec61883.h iec61883_cmp_connect -lraw1394 -lavc1394 -lrom1394 -liec61883