build: implement platform-agnostic build system

- Split Makefile into gmake.mk (Linux/Windows) and bmake.mk (BSD/macOS)
- Added config.mk for shared variables.
- Added build.sh and build.bat dispatchers.
- Retains old build behavior with dependency tracking.

build.sh

@@ -0,0 +1,11 @@
+#!/bin/sh
+
+OS=$(uname -s | tr '[:upper:]' '[:lower:]')
+
+if [ "$OS" = "linux" ]; then
+	MK_FILE="gmake.mk"
+else
+	MK_FILE="bmake.mk"
+fi
+
+make -f "$MK_FILE" "$@"