Use `CFLAGS` from the Makefile

[bavshin-f5]

Some compiler flags can affect the bindings or the binary in significant for us ways. At the very least, we should be able to find the preprocessor flags (-D) and pass to bindgen. That requires improving our Makefile parser and collecting more variables.

A practical example: I've been trying to enforce the alignment requirements by statically asserting that the NGX_ALIGNMENT the nginx binary was built with is large enough (improperly aligned pointers are UB in Rust, and some of the core:: APIs verify alignment), and found that it wasn't possible to detect that the alignment was overridden via auto/configure --with-cc-opt="-DNGX_ALIGNMENT=8".

[bavshin-f5]

Another practical example: currently we parse ALL_INCS for include lines, but completely ignore CFLAGS.

This is what we generate for the official FreeBSD packages:

Makefile snippet

CC =    cc
CFLAGS =  <...>  -I /usr/local/include
CPP =   cc -E
LINK =  $(CC)


ALL_INCS = -I src/core \
        -I src/event \
        -I src/event/modules \
        -I src/event/quic \
        -I src/os/unix \
        -I objs \
        -I src/http \
        -I src/http/modules \
        -I src/http/v2 \
        -I src/http/v3 \
        -I src/mail \
        -I src/stream

Unlike the more common configuration, -I /usr/local/include is in the CFLAGS and thus will be ignored.

  cargo:warning=In file included from $redacted/objs/$redacted/ngx-debug/build/nginx-sys-ac0da6f4fc2a5346/out/expand.c:3:
  cargo:warning=In file included from $redacted/src/core/ngx_core.h:75:
  cargo:warning=$redacted/src/core/ngx_regex.h:19:10: fatal error: 'pcre2.h' file not found
  cargo:warning=   19 | #include <pcre2.h>
  cargo:warning=      |          ^~~~~~~~~
  cargo:warning=1 error generated.

To reproduce this, use ./auto/configure --with-cc-opt='-I /usr/local/include' --with-ld-opt='-L /usr/local/lib' on FreeBSD. The flags override auto/'s probes for libs in /usr/local, preventing it from adding the path to ALL_INCS