Changed AM_CPPFLAGS to AM_CFLAGS and ran autogen.sh

AM_CPPFLAGS could get overwritten and making it not happen when using
libtool

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

AM_CPPFLAGS is ignored in preference to a per-executable (or per-library)
_CPPFLAGS variable if it is defined.