Xmodmap modifier used for meta

Question

I would like to set my Alt_L to trigger a meta key in emacs.

I've tried using xmodmap to set Alt_L as Mod1, Mod2, Mod3. But in emacs, when i try to type Alt_L + x (expecting M-x) it only prints "x".

I start X server with : setxkbmap -model pc105 -layout us,fr -option grp:alt_shift_toggle -option caps:none

When I print the modifiers with xmodmap -pm I get :

xmodmap:  up to 4 keys per modifier, (keycodes in parentheses):
shift       Shift_L (0x32),  Shift_R (0x3e)
lock

control     Control_L (0x25),  Control_L (0x42),  Control_R (0x69)
mod1        Alt_L (0x40),  Alt_R (0x6c),  Meta_L (0xcd)
mod2        Num_Lock (0x4d)
mod3

mod4        Super_L (0x85),  Super_R (0x86),  Super_L (0xce),  Hyper_L (0xcf)
mod5        ISO_Level3_Shift (0x5c),  Mode_switch (0xcb)

I feel the problem comes from emacs, not from X keyboard config.

How emacs handles those modifiers ?

EDIT: setxkbmap -query ouputs when I start X:

rules:      evdev                                                                                                                                                                                                   
model:      pc105                                                                                                                                                                                                   
layout:     us,fr                                                                                                                                                                                                   
options:    ctrl:nocaps,grp:alt_shift_toggle

Using xev when I type Alt_L then x, then release x and release Alt_L I get:

KeyPress event, serial 34, synthetic NO, window 0xc00001,                                                                                                                                                           
    root 0x1d9, subw 0x0, time 5433348, (-2,510), root:(640,512),                                                                                                                                                   
    state 0x0, keycode 64 (keysym 0xffe9, Alt_L), same_screen YES,                                                                                                                                                  
    XLookupString gives 0 bytes:                                                                                                                                                                                    
    XmbLookupString gives 0 bytes:                                                                                                                                                                                  
    XFilterEvent returns: False
KeyPress event, serial 34, synthetic NO, window 0xc00001,

    root 0x1d9, subw 0x0, time 5438940, (-2,510), root:(640,512),

    state 0x8, keycode 53 (keysym 0x78, x), same_screen YES,

    XLookupString gives 1 bytes: (78) "x"

    XmbLookupString gives 1 bytes: (78) "x"

    XFilterEvent returns: False
KeyRelease event, serial 34, synthetic NO, window 0xc00001,

    root 0x1d9, subw 0x0, time 5439004, (-2,510), root:(640,512),

    state 0x8, keycode 53 (keysym 0x78, x), same_screen YES,

    XLookupString gives 1 bytes: (78) "x"

    XFilterEvent returns: False
KeyRelease event, serial 34, synthetic NO, window 0xc00001,

    root 0x1d9, subw 0x0, time 5442460, (-2,510), root:(640,512),

    state 0x8, keycode 64 (keysym 0xffe9, Alt_L), same_screen YES,

    XLookupString gives 0 bytes:

    XFilterEvent returns: False