Task: Generate a C Project Makefile

You are building a C application called httpd for a project with the following layout:

httpd/
├── src/
│   ├── main.c
│   ├── server.c
│   └── router.c
├── include/
│   └── httpd.h
└── tests/
    └── test_router.c

Requirements:

• Compiler: GCC, user-overridable
• Compile flags: -Wall -Wextra -O2, user-overridable
• Output binary: build/httpd
• Dependencies: auto-detected via -MMD -MP flags
• Standard targets: all, install, clean, test, help
• install copies the binary to $(PREFIX)/bin (PREFIX defaults to /usr/local)
• help prints available targets using ## comment extraction
• Install prefix must be user-overridable with ?=
• Use the Modern GNU Make header (not POSIX-compatible header)
• Use order-only prerequisites for the build directory

Produce a single Makefile that satisfies all the requirements above. Include a validation checklist at the end as a comment block showing which items were verified.