Update BDWGC.

[fruffy]

• Use a more recent version of BDWGC, which also changes the folder structure.
• It looks like it uses some GNU-specific extensions, which is why _GNU_SOURCE is necessary for compilation.

Fixes #4857.

[fruffy]

@jkhsjdhjs Could try this PR and see whether it works for you?

[jkhsjdhjs]

Doesn't work for me due to

In file included from /home/jkhsjdhjs/aur/p4lang-p4c/src/p4c-fruffy-bdwgc_change/build/lib/CMakeFiles/p4ctoolkit.dir/Unity/unity_1_cxx.cxx:4:
/home/jkhsjdhjs/aur/p4lang-p4c/src/p4c-fruffy-bdwgc_change/lib/gc.cpp: In function ‘void setup_gc_logging()’:
/home/jkhsjdhjs/aur/p4lang-p4c/src/p4c-fruffy-bdwgc_change/lib/gc.cpp:321:22: error: invalid conversion from ‘void (*)(const char*, GC_word)’ {aka ‘void (*)(const char*, long unsigned int)’} to ‘GC_warn_proc’ {aka ‘void (*)(char*, long unsigned int)’} [-fpermissive]
  321 |     GC_set_warn_proc(&silent);
      |                      ^~~~~~~
      |                      |
      |                      void (*)(const char*, GC_word) {aka void (*)(const char*, long unsigned int)}
In file included from /usr/include/gc.h:2,
                 from /home/jkhsjdhjs/aur/p4lang-p4c/src/p4c-fruffy-bdwgc_change/lib/gc.cpp:32:
/usr/include/gc/gc.h:1409:38: note:   initializing argument 1 of ‘void GC_set_warn_proc(GC_warn_proc)’
 1409 | GC_API void GC_CALL GC_set_warn_proc(GC_warn_proc /* p */) GC_ATTR_NONNULL(1);
      |                                      ^~~~~~~~~~~~

[jkhsjdhjs]

I also just noticed that /usr/include/gc.h and /usr/include/gc_cpp.h simply contain

/* This file is installed for backward compatibility. */
#include <gc/gc.h>

and

/* This file is installed for backward compatibility. */
#include <gc/gc_cpp.h>

respectively.

So if you're already at it, maybe the other includes should be adjusted as well (if that's also compatible with the CMake Fetch Content build variant).

[jkhsjdhjs]

Builds fine for me now!