makefile: use ccache so it recompiles faster after a make clean

Signed-off-by: Dan Trifan <jimsth@gmail.com>
lib:makefile: fix compile error on GCC 4.7 with -O2/-O3 compilation

Fixes this:

lib/bitmap.c: In function 'bitmap_scnprintf':
lib/bitmap.c:401:1: internal compiler error: output_operand: invalid
shift operand

Signed-off-by: Dan Trifan <jimsth@gmail.com>