configure: Simplify avisynth check

configure

@@ -4217,8 +4217,8 @@ for func in $MATH_FUNCS; do
 done
 
 # these are off by default, so fail if requested and not available
-enabled avisynth          && { { check_header "avisynth/avisynth_c.h" && check_lib2 "windows.h" LoadLibrary; } ||
-                               { check_header "avxsynth/avxsynth_c.h" && check_lib2 "dlfcn.h" dlopen -ldl; } ||
+enabled avisynth          && { check_lib2 "avisynth/avisynth_c.h windows.h" LoadLibrary ||
+                               check_lib2 "avxsynth/avxsynth_c.h dlfcn.h" dlopen -ldl   ||
                                die "ERROR: LoadLibrary/dlopen not found, or avisynth header not found"; }
 enabled frei0r            && { check_header frei0r.h || die "ERROR: frei0r.h header not found"; }
 enabled gnutls            && require_pkg_config gnutls gnutls/gnutls.h gnutls_global_init