Let the user specify CPPFLAGS

This complements the work done in d4928ed, allowing the user to specify
the preprocessor flags with the CPPFLAGS environment variable. This is
useful for example to specify preprocessor macros with -D.

CFLAGS could be used instead, but CPPFLAGS is more correct and is expected
to be honored in some cases. For example, the helper scripts to build
Debian packages make use of CPPFLAGS, but the variable is currently
being ignored unless manually appended to CFLAGS.