mxmkmf: 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/mxmkmf - from package: lesstif-devel

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

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