build: Combine rules for object files.

build: Combine rules for object files.

Currently we have many similar recipes for compiling object files from
.c files that need non-standard CPPFLAGS. All rules are identical
except that different additional options to the $(CC) command are
required to build the object file. These additional options are
provided through various $(foo_cppflags) variables.

This commit avoids this duplication by adding the necessary flags to
CPPFLAGS instead. This way we can use the same rule for all object
files.