build: Fix libreaddir-rand to honor global CFLAGS

build: Fix libreaddir-rand to honor global CFLAGS

It's unfortunate that in automake one has to explicitly include the globa
`$(AM_CFLAGS)` if one sets `CFLAGS`, and similarly for other variables.

I'm trying to use `-fsanitize=address`, and not including it was
causing linker failures.

We also weren't inheriting the global warnings etc., so I had to fix a
decl-after-statement.

Closes: #351
Approved by: jlebon