This is the standard behavior for external libraries.
# these are off by default, so fail if requested and not available
enabled avisynth && { check_header avisynth/avisynth_c.h || die "ERROR: avisynth/avisynth_c.h header not found"; }
enabled avxsynth && require avxsynth "avxsynth/avxsynth_c.h dlfcn.h" dlopen -ldl
-enabled cuda && check_lib cuda.h cuInit -lcuda
+enabled cuda && require cuda cuda.h cuInit -lcuda
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
enabled libbs2b && require_pkg_config libbs2b bs2b.h bs2b_open