On OSX you can install XQuartz and libx11 an then compile it with (assuming you installed libx11 via brew)

  cc banks.c -o banks \
        -DIT=XK_Page_Up -DDT=XK_Page_Down \
        -DUP=XK_Up -DDN=XK_Down -DLT=XK_Left -DRT=XK_Right \
        -DCS=XK_Return -Ddt=0.02 \
        -std=c89 \
        -Wno-unsequenced -Wno-implicit-function-declaration -Wno-implicit-int -Wno-empty-body\
        -lm -lX11 -L/usr/local/Cellar/libx11/1.8.10/lib/ -I/usr/local/Cellar/libx11/1.8.10/include