regtest: some missing AM_CFLAGS and AM_CXXFLAGS

regtest: some missing AM_CFLAGS and AM_CXXFLAGS

In all cases, these caused problems for 32on64 builds.

In a few cases extra CFLAGS were needed but AM_CFLAGS not used.

And it seems as though autoconf needs explicit else branches. So

if FOO
test_CFLAGS = ${AM_CFLAGS} -hide-some-warnings
endif

doesn't set test_CFLAGS to the default when FOO is false.
(I'm sure I've seen that before, and forgotten).