fltk-config-i686: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/bin/fltk-config-i686 - from package: fltk-devel

You may install the required package with command yum install {package}

Error in other languages:
fltk-config-i686: bevel nie gevind nie
fltk-config-i686: comandă negăsită
fltk-config-i686: comando não encontrado
fltk-config-i686: commande introuvable
fltk-config-i686: command not found
fltk-config-i686: käsku ei ole
fltk-config-i686: không tìm thấy lệnh
fltk-config-i686: komanda nerasta
fltk-config-i686: Kommando nicht gefunden.
fltk-config-i686: kommandot finns inte
fltk-config-i686: komut yok
fltk-config-i686: nie znaleziono polecenia
fltk-config-i686: níor aimsíodh an t-ordú
fltk-config-i686: no se encontró la orden
fltk-config-i686: no s'ha trobat l'ordre
fltk-config-i686: opdracht niet gevonden
fltk-config-i686: parancs nem található
fltk-config-i686: perintah tidak ditemukan
fltk-config-i686: príkaz nenájdený
fltk-config-i686: příkaz nenalezen
fltk-config-i686: команда не найдена
fltk-config-i686: командата не е открита
fltk-config-i686: 命令找不到
fltk-config-i686: コマンドが見つかりません