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.

bmake.mk

@@ -0,0 +1,40 @@
+.include "config.mk"
+
+FULL_LIB     = lib${LIB_NAME}.so
+LDFLAGS_PLAT = -ldl
+CFLAGS_PLAT  = -fPIC
+CFLAGS_BASE  = -Wall -Wpedantic -I${INC_DIR} -std=c89 ${CFLAGS_PLAT}
+
+.PHONY: all debug release clean
+
+all: debug
+
+OBJS_DEBUG   = ${SRCS:S/^src\//obj\/debug\//:S/.c$/.o/}
+OBJS_RELEASE = ${SRCS:S/^src\//obj\/release\//:S/.c$/.o/}
+
+debug: ${BIN_DIR}/debug/${FULL_LIB}
+release: ${BIN_DIR}/release/${FULL_LIB}
+
+${BIN_DIR}/debug/${FULL_LIB}: ${OBJS_DEBUG}
+	@mkdir -p ${.TARGET:H}
+	${CC} -shared -o ${.TARGET} ${.ALLSRC} ${LDFLAGS_PLAT}
+
+${BIN_DIR}/release/${FULL_LIB}: ${OBJS_RELEASE}
+	@mkdir -p ${.TARGET:H}
+	${CC} -shared -o ${.TARGET} ${.ALLSRC} ${LDFLAGS_PLAT}
+
+.for _src in ${SRCS}
+obj/debug/${_src:T:R}.o: ${_src}
+	@mkdir -p ${.TARGET:H}
+	${CC} ${CFLAGS_BASE} -g -O0 -MMD -MP -c ${_src} -o ${.TARGET}
+
+obj/release/${_src:T:R}.o: ${_src}
+	@mkdir -p ${.TARGET:H}
+	${CC} ${CFLAGS_BASE} -O2 -MMD -MP -c ${_src} -o ${.TARGET}
+.endfor
+
+.dinclude "obj/debug/*.d"
+.dinclude "obj/release/*.d"
+
+clean:
+	rm -rf ${OBJ_DIR} ${BIN_DIR}