src/Makefile.am: We don't gain anything by setting AM_CFLAGS to C_OR_CXX_FLAGS
We were duplicating command-line flags un-necessarily, and it does not
stop the a build with the C++ compiler using CFLAGS, as was (in hindsight)
the original intention before commit
72cdf2ecef3361a00fbd0ee3e540f7383e62272c
where I changed a complete CFLAGS override to a AM_CFLAGS addition.
As we should not override CFLAGS in a Makefile (it prevents the user from
setting CFLAGS at "make" time), I'm removing this completely.