dwm build options:
CFLAGS   = -std=c99 -pedantic -Wall -Wno-deprecated-declarations -Os -I/usr/local/include -I/usr/local/include/freetype2 -D_DEFAULT_SOURCE -D_BSD_SOURCE -D_POSIX_C_SOURCE=200809L -DVERSION="6.4" -DXINERAMA
LDFLAGS  = -L/usr/local/lib -lX11 -lXinerama -lfontconfig -lXft
CC       = cc
cc -c -std=c99 -pedantic -Wall -Wno-deprecated-declarations -Os -I/usr/local/include -I/usr/local/include/freetype2 -D_DEFAULT_SOURCE -D_BSD_SOURCE -D_POSIX_C_SOURCE=200809L -DVERSION=\"6.4\" -DXINERAMA drw.c
cc -c -std=c99 -pedantic -Wall -Wno-deprecated-declarations -Os -I/usr/local/include -I/usr/local/include/freetype2 -D_DEFAULT_SOURCE -D_BSD_SOURCE -D_POSIX_C_SOURCE=200809L -DVERSION=\"6.4\" -DXINERAMA dwm.c
dwm.c:1549:31: error: use of undeclared identifier 'SA_NOCLDWAIT'
        sa.sa_flags = SA_NOCLDSTOP | SA_NOCLDWAIT | SA_RESTART;
                                     ^
dwm.c:1549:46: error: use of undeclared identifier 'SA_RESTART'
        sa.sa_flags = SA_NOCLDSTOP | SA_NOCLDWAIT | SA_RESTART;
                                                    ^
2 errors generated.
*** Error code 1

Stop.