It fails here because of missing xinerama support:
CFLAGS = -std=c99 -pedantic -Wall -Os -I. -I/usr/include -I/usr/X11R6/include
-DVERSION="5.0.1" -DXINERAMA
LDFLAGS = -Wl,-O1 -Wl,--sort-common -Wl,--as-needed -g -L/usr/lib -lc
-L/usr/X11R6/lib -lX11 -L/usr/X11R6/lib -lXinerama
CC = i686-pc-linux-gnu-gcc
creating config.h from config.def.h
CC dwm.c
dwm.c:43:37: error: X11/extensions/Xinerama.h: No such file or directory
dwm.c: In function 'updategeom':
dwm.c:1560: error: 'XineramaScreenInfo' undeclared (first use in this function)
dwm.c:1560: error: (Each undeclared identifier is reported only once
dwm.c:1560: error: for each function it appears in.)
dwm.c:1560: error: 'info' undeclared (first use in this function)
dwm.c:1563: warning: implicit declaration of function 'XineramaIsActive'
dwm.c:1564: warning: implicit declaration of function 'XineramaQueryScreens'
make: *** [dwm.o] Error 1
from config.mk:
| # Xinerama, comment if you don't want it
| XINERAMALIBS = -L${X11LIB} -lXinerama
| XINERAMAFLAGS = -DXINERAMA
