Never override CFLAGS in a makefile

This prevents the user from specifying his own desired CFLAGS.

https://bugzilla.gnome.org/show_bug.cgi?id=741223
Don't set AM_CFLAGS in configure.ac

This makes it impossible to use AM_CFLAGS, a useful variable, without
manually overwriting the value given in configure.ac. Instead, just
add WARN_CFLAGS to AM_CFLAGS everywhere.

Also, stop setting several other variables with empty values.

https://bugzilla.gnome.org/show_bug.cgi?id=741223
configure.ac: remove useless uses of AC_SUBST