fltk-config-x86_64: 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-x86_64 - from package: fltk-devel

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

Error in other languages:
fltk-config-x86_64: bevel nie gevind nie
fltk-config-x86_64: comandă negăsită
fltk-config-x86_64: comando não encontrado
fltk-config-x86_64: commande introuvable
fltk-config-x86_64: command not found
fltk-config-x86_64: käsku ei ole
fltk-config-x86_64: không tìm thấy lệnh
fltk-config-x86_64: komanda nerasta
fltk-config-x86_64: Kommando nicht gefunden.
fltk-config-x86_64: kommandot finns inte
fltk-config-x86_64: komut yok
fltk-config-x86_64: nie znaleziono polecenia
fltk-config-x86_64: níor aimsíodh an t-ordú
fltk-config-x86_64: no se encontró la orden
fltk-config-x86_64: no s'ha trobat l'ordre
fltk-config-x86_64: opdracht niet gevonden
fltk-config-x86_64: parancs nem található
fltk-config-x86_64: perintah tidak ditemukan
fltk-config-x86_64: príkaz nenájdený
fltk-config-x86_64: příkaz nenalezen
fltk-config-x86_64: команда не найдена
fltk-config-x86_64: командата не е открита
fltk-config-x86_64: 命令找不到
fltk-config-x86_64: コマンドが見つかりません