There is a regression between version 5.37 and version 5.40, causing the
build to fail when building outside the source directory.

```
$ make -j1
[…]
make[1]: Leaving directory '/dev/shm/bee-root/xscreensaver/xscreensaver-5.40-0/build/hacks/images'
make[1]: Entering directory '/dev/shm/bee-root/xscreensaver/xscreensaver-5.40-0/build/hacks'
gcc -pedantic -Wall -Wstrict-prototypes -Wnested-externs -Wmissing-prototypes -Wno-overlength-strings -Wdeclaration-after-statement -std=c89 -U__STRICT_ANSI__ -c -I. -I/dev/shm/bee-root/xscreensaver/xscreensaver-5.40-0/source/hacks -I/dev/shm/bee-root/xscreensaver/xscreensaver-5.40-0/source/hacks/../utils -I..  -pthread -I/usr/include/gtk-2.0 -I/usr/lib/gtk-2.0/include -I/usr/include/atk-1.0 -I/usr/include/cairo -I/usr/include/gdk-pixbuf-2.0 -I/usr/include/pango-1.0 -I/usr/include/glib-2.0 -I/usr/lib/glib-2.0/include -I/usr/include/fribidi -I/usr/include/harfbuzz -I/usr/include/freetype2 -I/usr/include/libpng16 -I/usr/include/pixman-1 -I/usr/include/libxml2 -I/usr/include/libglade-2.0   -pthread -I/usr/include/gdk-pixbuf-2.0 -I/usr/include/libpng16 -I/usr/include/glib-2.0 -I/usr/lib/glib-2.0/include   -I/usr/include/freetype2 -I/usr/include/libpng16 -I/usr/include/harfbuzz -I/usr/include/glib-2.0 -I/usr/lib/glib-2.0/include   -I/usr/include -DSTANDALONE -DHAVE_CONFIG_H  -g -O2  /dev/shm/bee-root/xscreensaver/xscreensaver-5.40-0/source/hacks/attraction.c
gcc -pedantic -Wall -Wstrict-prototypes -Wnested-externs -Wmissing-prototypes -Wno-overlength-strings -Wdeclaration-after-statement -std=c89 -U__STRICT_ANSI__ -c -I. -I/dev/shm/bee-root/xscreensaver/xscreensaver-5.40-0/source/hacks -I/dev/shm/bee-root/xscreensaver/xscreensaver-5.40-0/source/hacks/../utils -I..  -pthread -I/usr/include/gtk-2.0 -I/usr/lib/gtk-2.0/include -I/usr/include/atk-1.0 -I/usr/include/cairo -I/usr/include/gdk-pixbuf-2.0 -I/usr/include/pango-1.0 -I/usr/include/glib-2.0 -I/usr/lib/glib-2.0/include -I/usr/include/fribidi -I/usr/include/harfbuzz -I/usr/include/freetype2 -I/usr/include/libpng16 -I/usr/include/pixman-1 -I/usr/include/libxml2 -I/usr/include/libglade-2.0   -pthread -I/usr/include/gdk-pixbuf-2.0 -I/usr/include/libpng16 -I/usr/include/glib-2.0 -I/usr/lib/glib-2.0/include   -I/usr/include/freetype2 -I/usr/include/libpng16 -I/usr/include/harfbuzz -I/usr/include/glib-2.0 -I/usr/lib/glib-2.0/include   -I/usr/include -DSTANDALONE -DHAVE_CONFIG_H  -g -O2  /dev/shm/bee-root/xscreensaver/xscreensaver-5.40-0/source/hacks/screenhack.c
gcc -pedantic -Wall -Wstrict-prototypes -Wnested-externs -Wmissing-prototypes -Wno-overlength-strings -Wdeclaration-after-statement -std=c89 -U__STRICT_ANSI__ -c -I. -I/dev/shm/bee-root/xscreensaver/xscreensaver-5.40-0/source/hacks -I/dev/shm/bee-root/xscreensaver/xscreensaver-5.40-0/source/hacks/../utils -I..  -pthread -I/usr/include/gtk-2.0 -I/usr/lib/gtk-2.0/include -I/usr/include/atk-1.0 -I/usr/include/cairo -I/usr/include/gdk-pixbuf-2.0 -I/usr/include/pango-1.0 -I/usr/include/glib-2.0 -I/usr/lib/glib-2.0/include -I/usr/include/fribidi -I/usr/include/harfbuzz -I/usr/include/freetype2 -I/usr/include/libpng16 -I/usr/include/pixman-1 -I/usr/include/libxml2 -I/usr/include/libglade-2.0   -pthread -I/usr/include/gdk-pixbuf-2.0 -I/usr/include/libpng16 -I/usr/include/glib-2.0 -I/usr/lib/glib-2.0/include   -I/usr/include/freetype2 -I/usr/include/libpng16 -I/usr/include/harfbuzz -I/usr/include/glib-2.0 -I/usr/lib/glib-2.0/include   -I/usr/include -DSTANDALONE -DHAVE_CONFIG_H  -g -O2  /dev/shm/bee-root/xscreensaver/xscreensaver-5.40-0/source/hacks/fps.c
gcc -pedantic -Wall -Wstrict-prototypes -Wnested-externs -Wmissing-prototypes -Wno-overlength-strings -Wdeclaration-after-statement -std=c89 -U__STRICT_ANSI__ -L/usr/lib -o attraction attraction.o  screenhack.o fps.o ../utils/resources.o ../utils/visual.o ../utils/usleep.o ../utils/yarandom.o ../utils/utf8wc.o ../utils/font-retry-xft.o  ../utils/hsv.o ../utils/colors.o ../utils/spline.o     -lSM -lICE -lXft   -lXt -lX11 -lXmu -lXext  -lm
make[1]: *** No rule to make target '/dev/shm/bee-root/xscreensaver/xscreensaver-5.40-0/source/hacks/images/gen/som_png.h', needed by 'blitspin.o'.  Stop.
make[1]: Leaving directory '/dev/shm/bee-root/xscreensaver/xscreensaver-5.40-0/build/hacks'
make: *** [Makefile:25: default] Error 5
```