build: fix iconv detection on some systems.

This at least fix the build on CentOS.

configure

@@ -3765,7 +3765,7 @@ check_func  getopt
 check_func  getrusage
 check_struct "sys/time.h sys/resource.h" "struct rusage" ru_maxrss
 check_func  gettimeofday
-check_func  iconv || check_lib2 iconv.h iconv -liconv
+check_func_headers iconv.h iconv || check_lib2 iconv.h iconv -liconv
 check_func  inet_aton $network_extralibs
 check_func  isatty
 check_func  localtime_r