The <iomanip> header is already checked; <iomanip.h> is pre-C++98 alias.

--- id3lib-3.8.3/configure.in.old	2003-03-02 00:23:00.000000000 +0000
+++ id3lib-3.8.3/configure.in	2018-03-25 06:43:58.270837035 +0000
@@ -227,7 +227,6 @@
 )
 AC_CHECK_HEADERS(               \
   string                        \
-  iomanip.h                     \
   ,,AC_MSG_ERROR([Missing a vital header file for id3lib])
 )
 
--- id3lib-3.8.3/configure.old	2003-03-02 00:23:00.000000000 +0000
+++ id3lib-3.8.3/configure	2018-03-25 06:44:02.260837054 +0000
@@ -10296,7 +10296,6 @@
 
 for ac_header in \
   string                        \
-  iomanip.h                     \
 
 do
 as_ac_Header=`echo "ac_cv_header_$ac_header" | $as_tr_sh`