Fix variant suffix resolution and binary target name in makefile

[tobfal]

Problem

ifeq (LARGE_BIOMES, 1) compares the literal string against "1", so it never matches and BIN_SUFFIX is always "sb" regardless of the options.

The Windows target is named main.exe while the recipe writes to $@-$(BIN_SUFFIX). Target and output never match, so make rebuilds everything on every invocation, and if a file named main.exe happens to exist it reports "Nothing to be done" and skips the build entirely.

Changes

• resolve the variables in both conditionals
• concatenate BIN_SUFFIX without spaces and append "u" last, so the
  existing main-sb* / main-lb* gitignore patterns cover all four variants
• introduce BIN and use it for both the target and -o $@
• clean removes main-*.exe

Verified on Windows