allow replacing the malloc with LC_PRELOAD

Preloading a different memory alloctor, for example [hardened_malloc](github.com/grapheneos/hardened_malloc) is possible but disabled in upstream firefox.

This caused [this bug](secureblue/secureblue#171)

The build flag was already enabled in Fedora: [Issue](https://bugzilla.redhat.com/show_bug.cgi?id=2260766) [Changelog]( https://bodhi.fedoraproject.org/updates/FEDORA-2024-7af93e63f3)

This only does something when the user uses LD_PRELOAD to preload a different memory allocator, i.e. nothing on most systems. Other systems may chose to not yet compile glibc/musl with that memory allocator, so they use this trick to still allow using a different memory allocator. This also allows them to use the regular malloc as a "fallback", which may be wanted.

This flag is not compatible with `--disable-jemalloc` which does not compile jemalloc at all. That flag create a perfectly usable binary, but only on systems where a different one, e.g. hardened_malloc is preloaded.

For the future it could be possible to do this in Librewolf, but this would require lots of testing, I left the comment in there anyways.

Author: wannabetechie946

assets/mozconfig.new

@@ -11,6 +11,9 @@ ac_add_options --enable-jxl
 ac_add_options --enable-optimize
 ac_add_options --enable-release
 ac_add_options --enable-rust-simd
+# allow replacing malloc manually, will not affect regular systems
+# not compatible with ac_add_options --disable-jemalloc, which only works when preloading custom malloc
+ac_add_options --enable-replace-malloc
 
 ac_add_options --with-app-name=hyperfox
 ac_add_options --with-branding=browser/branding/librewolf