Add warning flags to the DEBUG_CFLAGS too.

We don't want to have warnings conditional to extreme optimisations:
if the compiler supports them, they should be enabled.

The reason why they were conditional is that the code testing for GCC
presence enabled both warnings and optimisations, and the warnings
were carried over when the optimisations were made optional.

When passing CC="ccache gcc", we don't want to quote "$CC" calls,
otherwise the command won't be found.

former.

The CC_CHECK_CFLAGS macro is already present in attributes.m4, so no
need to keep the old version we had around, especially since the new
one caches its results too.

equivalent.

I wanted this because I'm going to reuse attribute.m4 with that macro
for a couple other projects.

This also removes AC_TRY_LDFLAGS since it's not used any longer
(replaced by CC_CHECK_LDFLAGS, which also caches results).

This adds variants of CC_CHECK_CFLAGS that append the flags
automatically, and a few comments.

autoconf already adds the "int main() { ...; return 0; }" statements itself,
and so adding them twice chokes stricter compilers.