boehmgc: drop musl flags, per upstream's note (!) (#157625)

0938316e05 (commitcomment-63005097)

Resulting binaries are identifical according to diffoscope
(other than embedded /nix/store/.../ paths).

FWIW, going back through versions we've packaged, the most recent
instance where these flags produced differing outputs was in 7.6.8
(comparing $out's only, FWIW).

pkgs/development/libraries/boehm-gc/default.nix

@@ -19,10 +19,6 @@ stdenv.mkDerivation rec {
   outputs = [ "out" "dev" "doc" ];
   separateDebugInfo = stdenv.isLinux && stdenv.hostPlatform.libc != "musl";
 
-  preConfigure = lib.optionalString (stdenv.hostPlatform.libc == "musl") ''
-    export NIX_CFLAGS_COMPILE+=" -D_GNU_SOURCE -DUSE_MMAP -DHAVE_DL_ITERATE_PHDR"
-  '';
-
   # boehm-gc whitelists GCC threading models
   patches = lib.optional stdenv.hostPlatform.isMinGW ./mcfgthread.patch;