Log Message:
On progressive Linuxen (Arch here), CPPFLAGS pre-filled with
-D_FORTIFY_SOURCE=2 -Werror will make configure tests fail on
pre-processor warnings, and break the build. Make sure we set CPP to
something that works.

This patch should go upstream into configure.in, but it is really the
autoconf AC_PROG_CPP macro definition that needs to be amended.