Makefile: Honor CPPFLAGS also when creating dependencies.

Makefile: Honor CPPFLAGS also when creating dependencies.

Makefile.deps is created using a cc -MM -MG command which might need
the CPPFLAGS provided by the user, just as the ordinary cc commands
for creating an object file.

For example, the user might have passed -I$HOME/include, since
libosl was installed in $HOME. In this case we need to pass
this to the cc command that creates the Makefile as well.