Add automatic dependency building to Makefile

- Add secp256k1 auto-building rule for x64 target
- Fix customer build errors with missing secp256k1/.libs/libsecp256k1.a
- Force clean rebuild when secp256k1 library is missing
- Customers can now run 'make' directly without manual dependency building
- Maintains existing ARM64 cross-compilation auto-building functionality

Makefile

@@ -43,6 +43,21 @@ default: $(STATIC_LIB) $(ARM64_STATIC_LIB)
 # Build all targets (static only)
 all: $(STATIC_LIB) $(ARM64_STATIC_LIB) examples
 
+# Build secp256k1 for x86_64
+$(SECP256K1_LIB): secp256k1/configure
+	@echo "Building secp256k1 for x86_64..."
+	@cd secp256k1 && \
+		if [ ! -f .libs/libsecp256k1.a ]; then \
+			echo "Cleaning and configuring secp256k1..."; \
+			make distclean >/dev/null 2>&1 || true; \
+			./configure --enable-module-schnorrsig --enable-module-ecdh --enable-experimental --disable-shared --enable-static --with-pic; \
+			echo "Building secp256k1 library..."; \
+			make -j$(shell nproc 2>/dev/null || echo 4); \
+		else \
+			echo "secp256k1 library already exists, skipping build"; \
+		fi
+	@echo "x86_64 secp256k1 library built successfully"
+
 # Static library - includes secp256k1 and OpenSSL objects for self-contained library
 $(STATIC_LIB): $(LIB_OBJECTS) $(SECP256K1_LIB)
 	@echo "Creating self-contained static library: $@"