use $(CC) instead of $(CPP) to generate .depend files

use $(CC) instead of $(CPP) to generate .depend files

Use $(CC) instead of $(CPP) to generate .depend file because '$(CPP)
-M' call does not support multiple input files.  This avoid the
following error:

make[1]: Entering directory `/opt/br/output/build/newt-0.51.0'
/opt/br/output/host/usr/bin/arm-none-linux-gnueabi-cpp -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -D_GNU_SOURCE -M newt.c button.c form.c checkbox.c entry.c label.c listbox.c scrollbar.c textbox.c scale.c grid.c windows.c buttonbar.c checkboxtree.c > .depend
arm-none-linux-gnueabi-cpp: too many input files
make[1]: *** [depend] Error 1

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>