Improve build tracking for C source changes

- Add @external_resource declarations for all C files so mix compile
  detects changes without needing 'mix clean' first
- Add -MMD -MP flags to Makefile for automatic header dependency tracking
- Clean .d files in 'make clean' target

Author: mjc

Makefile

@@ -138,7 +138,10 @@ endif
 
 $(BUILD)/%.o: c_src/%.c
 	@echo " CC $(notdir $@)"
-	$(CC) -c $(ERL_CFLAGS) $(CFLAGS) -o $@ $<
+	$(CC) -c $(ERL_CFLAGS) $(CFLAGS) -MMD -MP -o $@ $<
+
+# Include dependency files for automatic header tracking
+-include $(OBJ:.o=.d)
 
 $(LIB_NAME): $(OBJ)
 	@echo " LD $(notdir $@)"
@@ -152,7 +155,7 @@ $(PREFIX) $(BUILD):
 	mkdir -p $@
 
 clean:
-	$(RM) $(LIB_NAME) $(ARCHIVE_NAME) $(OBJ)
+	$(RM) $(LIB_NAME) $(ARCHIVE_NAME) $(OBJ) $(OBJ:.o=.d)
 
 .PHONY: all clean
 

lib/exqlite/sqlite3_nif.ex

@@ -4,6 +4,12 @@ defmodule Exqlite.Sqlite3NIF do
   should be avoided unless you are aware of what you are doing.
   """
 
+  # Track C source files so Mix recompiles when they change
+  @external_resource "c_src/sqlite3_nif.c"
+  @external_resource "c_src/sqlite3.c"
+  @external_resource "c_src/sqlite3.h"
+  @external_resource "c_src/sqlite3ext.h"
+
   @compile {:autoload, false}
   @on_load {:load_nif, 0}