examples/c: rebuild on header changes via -MMD

[Dhanya16]

$(wildcard %.h) does not expand the pattern stem, so local headers were never prerequisites. Generate dependency files with -MMD/-MP, and use -MT for BPF objects so deps attach to the final .bpf.o rather than the temporary .tmp.bpf.o.

Fixes #275

Test plan

• touch bootstrap.h && make bootstrap rebuilds BPF/skel/CC/binary
• touch bootstrap.h && make .output/bootstrap.bpf.o rebuilds
• .d files list bootstrap.h; BPF .d targets .bpf.o (not .tmp)

Before:

$ touch bootstrap.bpf.c
$ make bootstrap
  BPF      .output/bootstrap.bpf.o
  GEN-SKEL .output/bootstrap.skel.h
  CC       .output/bootstrap.o
  BINARY   bootstrap
$ touch bootstrap.c
$ make bootstrap
  CC       .output/bootstrap.o
  BINARY   bootstrap
$ touch bootstrap.h
$ make bootstrap
make: 'bootstrap' is up to date.
$ touch bootstrap.h
$ make .output/bootstrap.bpf.o
make: '.output/bootstrap.bpf.o' is up to date.

After:

$ touch bootstrap.h
$ make bootstrap
  BPF      .output/bootstrap.bpf.o
  GEN-SKEL .output/bootstrap.skel.h
  CC       .output/bootstrap.o
  BINARY   bootstrap
$ touch bootstrap.h
$ make .output/bootstrap.bpf.o
  BPF      .output/bootstrap.bpf.o

[danielocfb]

Looks good to me, thanks for the fix.

[danielocfb]

Btw, I think the CMake path has a similar issue, in case you feel like looking at that as well.

[Dhanya16]

Thanks! I'll investigate the CMake side and send a follow-up PR.