build-sys: Use AM_CPPFLAGS instead of AM_CFLAGS for preprocessor arguments

build-sys: Use AM_CPPFLAGS instead of AM_CFLAGS for preprocessor arguments

As the automake documentation says:

AM_CPPFLAGS: The contents of this variable are passed to every compilation
that invokes the C preprocessor; it is a list of arguments to the preprocessor.
For instance, -I and -D options should be listed here

AM_CFLAGS: This is the variable the Makefile.am author can use to pass in
additional C compiler flags.

http://www.gnu.org/software/automake/manual/html_node/Program-Variables.html