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.

config.mk

@@ -0,0 +1,11 @@
+SRC_DIR = src
+INC_DIR = include
+OBJ_DIR = obj
+BIN_DIR = bin
+
+LIB_NAME = stk
+
+SRCS = src/module.c \
+       src/platform.c \
+       src/stk.c \
+       src/stk_log.c