Fix -filter flag and extend stdout streaming to returns tables

[cjohnson-confluent]

What was broken

The -filter flag (which streams generated data to stdout instead of writing
files) was silently broken by the v2.3.0 upstream import (12caac0), which
reverted three fixes Greg Rahn had originally landed in 7992dbb:

1. params.h: Option was named _FILTER but is_set() searches for
   "FILTER" using prefix matching — it never matched, so filter mode never
   activated for any table.
2. print.c (print_start): fpOutfile = pTdef->outfile ran
   unconditionally, overwriting the stdout assignment with NULL on every
   row.
3. w_store_sales.c: Returns generation ran even in filter mode, writing
   interleaved sales and returns rows to stdout in a single pass.

What's new

• Restores all three fixes above
• Extends -filter support to the three returns tables (store_returns,
  catalog_returns, web_returns). Because returns are generated as a side
  effect of their parent sales table, a g_filter_tabid global tracks the
  target table; driver.c redirects child table requests to the parent
  generator, and print_start routes only the target to stdout, suppressing
  the parent's output to /dev/null.
• Auto-detects OS in makefile (Darwin → MACOS, else LINUX) so the
  same build works on macOS and Linux without a manual OS= override.
• Adds compiler flags for compatibility with modern GCC/clang on K&R-style C
  (-Wno-implicit-int -Wno-deprecated-non-prototype -fcommon).
• Adds a clear error when -filter is used without specifying -table <name>.

Verification

Output verified byte-for-byte identical to non-filter mode for all tables.

Usage

./dsdgen -scale N -table store_sales    -filter -quiet 2>/dev/null | gzip > store_sales.gz
./dsdgen -scale N -table store_returns  -filter -quiet 2>/dev/null | gzip > store_returns.gz
# same pattern for catalog_sales/returns, web_sales/returns